Under the Foreign Acquisitions and Takeovers Act 1975, certain 'significant actions' can be prohibited by the Treasury if considered to be contrary to the 'national interest'. These include actions to acquire interests in securities, assets or Australian land, and actions taken in relation to entities (being corporations and unit trusts) and businesses, that have a connection to Australia. In general, a change in control and meeting a monetary threshold test are required for an action to be significant. The national interest, and what would be contrary to it, is not defined in the Act or the Foreign Acquisitions and Takeovers Regulation 2015. Assessments are made on a case-by-case basis. However, a Foreign Investment Policy document states that the Government typically considers the following factors when assessing foreign investment proposals: national security; competition; other Australian government policies, including tax; impact on the economy and the community and the character of the investor. 
Additionally, any 'notifiable action' must be notified to the Treasury before the action can be taken. A notifiable action is a proposed action by a foreign person to acquire a direct interest in an Australian entity or Australian business that is an agribusiness, to acquire a substantial interest (generally at least 20%) in an Australian entity or to acquire an interest in Australian land. Generally, the action is notifiable when a monetary threshold test is met. The monetary screening thresholds concern either the amount paid for an interest or the value of an entity or assets. The relevant threshold depends on the type of investor and type of action taken. Generally, a higher threshold applies for investors from certain countries with which Australia has free trade agreements when these investors invest in non-sensitive businesses. Sensitive businesses, as defined in the Regulation, include transport, media and telecommunications businesses, among others. From 1 January 2021, the temporary $0 screening thresholds, introduced in 2020 in response to the COVID-19 pandemic, no longer apply. Instead, the prior tiered threshold regime is reinstated. 
From 1 January 2021, amendments to the Act and Regulation introduced, among other changes, the possibility to screen certain investments under a narrower 'national security test' when these investments do not meet the monetary and control thresholds of the already existing mechanisms but nonetheless pose national security concerns. For the purposes of the national security test, two new categories of actions, 'notifiable national security actions' and 'revieweable national security actions', and the concept of 'national security business', were introduced in the Act and Regulation. Examples of national security businesses include responsible entities and direct interest holders of critical infrastructure assets, as defined in the Security of Critical Infrastructure Act 2018, and carriers or nominated carriage service providers to which the Telecommunications Act 1997 applies.
"4 Dictionary of definitions
 In this Act: [...]
foreign person means:
 (a) an individual not ordinarily resident in Australia; or
 (b) a corporation in which an individual not ordinarily resident in Australia, a foreign corporation or a foreign government holds a substantial interest; or
 (c) a corporation in which 2 or more persons, each of whom is an individual not ordinarily resident in Australia, a foreign corporation or a foreign government, hold an aggregate substantial interest; or
 (d) the trustee of a trust in which an individual not ordinarily resident in Australia, a foreign corporation or a foreign government holds a substantial interest; or
 (e) the trustee of a trust in which 2 or more persons, each of whom is an individual not ordinarily resident in Australia, a foreign corporation or a foreign government, hold an aggregate substantial interest; or
 (f) a foreign government; or
 (g) any other person, or any other person that meets the conditions, prescribed by the regulations.[ ...]
national security business has the meaning prescribed by the regulations. [...]
substantial interest: a person holds a substantial interest in an entity, trust or unincorporated limited partnership if:
 (a) for an entity or unincorporated limited partnership—the person holds an interest of at least 20% in the entity or partnership; or
 (b) for a trust (including a unit trust)—the person, together with any one or more associates, holds a beneficial interest in at least 20% of the income or property of the trust.
[...]
26 Meaning of sensitive business
 (1) A business is a sensitive business if the business meets the conditions specified in the regulations.
 [...]
40 Meaning of significant action—entities
 (1) An action is a significant action if the conditions in this section are met.
First condition—kinds of action
 (2) The first condition is that the action is any of the following:
 (a) to acquire a direct interest in an Australian entity that is an agribusiness;
 (b) to acquire interests in securities in an entity;
 (c) to issue securities in an entity; [...]
Second condition—threshold
 (3) The second condition is that the threshold test is met in relation to the entity (see Subdivision B of Division 4).
Third condition—kinds of entities covered
 (4) The third condition (except if paragraph (2)(a) applies) is that the entity is the following:
 (a) for an acquisition of interests in shares or an issue of shares:
 (i) a corporation that is a relevant entity that carries on an Australian business, whether alone or together with one or more other persons; or
 (ii) the holding entity of such a corporation;
 (b) for any other action relating to a corporation:
 (i) an Australian corporation that carries on an Australian business, whether alone or together with one or more other persons; or
 (ii) the holding entity (other than a foreign corporation) of such a corporation;
 (c) for an action relating to a unit trust:
 (i) an Australian unit trust; or
 (ii) a holding entity of an Australian unit trust.
Last condition—action taken by a foreign person
 (5) If paragraph (2)(a) applies, the last condition is that the action is or is to be taken by a foreign person.
Last condition—action results in change in control
 (6) If any of paragraphs (2)(b) to (e) apply, the last condition is that there would be or has been a change in control of the entity as a result of the action (see Subdivision C of Division 4).
 (7) However, the last condition in subsection (6) is not required to be met if:
 (a) paragraph (2)(b) applies and the action is or is to be taken by a foreign person who controls the entity (whether alone or together with one or more associates) immediately before the action is or is to be taken; or
 (b) paragraph (2)(c) applies and a foreign person controls the entity (whether alone or together with one or more associates) immediately before the action is or is to be taken.
[...]
41 Meaning of significant action—businesses
 (1) An action is a significant action if the conditions in this section are met.
First condition—kinds of action
 (2) The first condition is that the action is any of the following:
 (a) to acquire a direct interest in an Australian business that is an agribusiness;
 (b) to acquire interests in assets of an Australian business; [...]
Second condition—threshold
 (3) The second condition is that the threshold test is met in relation to the Australian business (see Subdivision B of Division 4).
Third condition—action taken by a foreign person
 (4) If paragraph (2)(a) applies, the third condition is that the action is or is to be taken by a foreign person.
Third condition—action results in change in control
 (5) If paragraph (2)(b) or (c) applies, the third condition is that there would be or has been a change in control of the business as a result of the action (see Subdivision C of Division 4).
(6) However, the third condition in subsection (5) is not required to be met if:
 (a) paragraph (2)(b) applies; and
 (b) the action is or is to be taken by a foreign person who controls the business (whether alone or together with one or more associates) immediately before the action is or is to be taken.
[...]
44 Meaning of significant action—actions prescribed by the regulations
(1) The regulations may provide that a specified action is a significant action (including by providing that a specified action is a significant action if specified conditions are met).
[...]
47 Meaning of notifiable action—general
(1) An action is a notifiable action if the conditions in this section are met.
First condition—kinds of action
(2) The first condition is that the action is any of the following:
(a) to acquire a direct interest in:
(i) an Australian entity that is an agribusiness; or
(ii) an Australian business that is an agribusiness;
(b) to acquire a substantial interest in an Australian entity;
(c) to acquire an interest in Australian land.
Second condition—threshold
(3) The second condition is that the threshold test is met in relation to the entity, business or land (see Subdivision B of Division 4).
(4) If subparagraph (2)(a)(i) or paragraph (b) applies, the third condition is that the entity is:
(a) an Australian corporation that carries on an Australian business, whether alone or together with one or more other persons; or
(b) an Australian unit trust; or
(c) an Australian entity that is the holding entity of an entity mentioned in paragraph (a) or (b).
Last condition—action taken by a foreign person
(5) The last condition (whether paragraph (2)(a), (b) or (c) applies) is that the action is or is to be taken by a foreign person.
[...]
48 Meaning of notifiable action—actions prescribed by the regulations
 (1) The regulations may provide that a specified action is a notifiable action (including by providing that a specified action is a notifiable action if specified conditions are met).
 [...]
51 The threshold test for entities and businesses
 The threshold test is met in relation to an entity or business for a significant action mentioned in column 1 of the following table if the value mentioned in column 2 of the table is more than the value prescribed by the regulations. 
When the threshold test is met
Item | Column 1: Significant Action | Column 2: Value
1 | A person acquiring a direct interest in an Australian entity or Australian business an agribusiness  | The total of the following: (a) the value of the consideration for the acquisition; (b) the total value of the other interests held by the person,  alone or together with one or more associates:(i) in the entity or business; or (ii) previously acquired from the entity or business
2 | Acquiring interests in securities in an entity, or issuing securities in an entity| The higher of the following: (a) the total asset value for the entity; (b) the total issued securities value for the entity [...] 
4 | Acquiring interests in the assets of an Australian business | The value of the consideration for the acquisition
[...]
55B Meaning of notifiable national security action
 (1) An action is a notifiable national security action if the action is taken, or proposed to be taken, by a foreign person and the action is any of the following:
 (a) to start a national security business;
 (b) to acquire a direct interest in a national security business;
 (c) to acquire a direct interest in an entity that carries on a national security business;
 (d) to acquire an interest in Australian land that, at the time of acquisition, is national security land;
 (e) to acquire a legal or equitable interest in an exploration tenement in respect of Australian land that, at the time of acquisition, is national security land.
 (2) For the purposes of paragraph (1)(e), a person acquires a legal or equitable interest in an exploration tenement even if:
 (a) the person has previously acquired an interest in an exploration tenement; or
 (b) the interest is an increase in the amount of an existing interest of the person in an exploration tenement.
 (3) The regulations may provide, in relation to exemption certificates in force under regulations made for the purposes of section 63, that an action of a specified kind is not a notifiable national security action.
[...]
55D Meaning of reviewable national security action—entities
Meanings
 (1) An action is a reviewable national security action if:
 (a) the action is taken, or proposed to be taken, by a person and the action is to acquire an interest of any percentage in an entity; and
 (b) as a result of the action or proposed action:
 (i) a foreign person acquires, or will acquire, a direct interest in the entity and that acquisition, or proposed acquisition, is not a significant action, notifiable action or notifiable national security action; or
 (ii) a foreign person will be in a position, or more of a position, to influence or participate in the central management and control of the entity; or
 (iii) a foreign person will be in a position, or more of a position, to influence, participate in or determine the policy of the entity; and
 (c) the action is or was not otherwise a significant action, a notifiable action or a notifiable national security action.
 (2) An action is also a reviewable national security action if:
 (a) the action is taken, or proposed to be taken, by a person and the action is:
 (i) to issue securities in an entity; or
 (ii) to enter an agreement relating to the affairs of an entity and under which one or more senior officers of the entity will be under an obligation to act in accordance with the directions, instructions or wishes of a foreign person who holds a direct interest in the entity; or
 (iii) to alter a constituent document of an entity as a result of which one or more senior officers of the entity will be under an obligation to act in accordance with the directions, instructions or wishes of a foreign person who holds a direct interest in the entity; and
 (b) in the case to which subparagraph (a)(i) applies—as a result of the action or proposed action:
 (i) a foreign person acquires, or will acquire, a direct interest in the entity and that acquisition, or proposed acquisition, is not a significant action, notifiable action or notifiable national security action; or
 (ii) a foreign person will be in a position, or more of a position, to influence or participate in the central management and control of the entity; or
 (iii) a foreign person will be in a position, or more of a position, to influence, participate in or determine the policy of the entity; and
 (c) the action is or was not otherwise a significant action, a notifiable action or a notifiable national security action.
Definitions of entity
 (3) For the purposes of subsection (1), the entity must be:
 (a) for an acquisition of interests in shares:
 (i) a corporation that carries on an Australian business, whether alone or together with one or more other persons; or
 (ii) the holding entity of such a corporation; or
 (b) for any other action relating to a corporation:
 (i) an Australian corporation that carries on an Australian business, whether alone or together with one or more other persons; or
 (ii) the holding entity (other than a foreign corporation) of such a corporation; or
 (c) for an action relating to a unit trust:
 (i) an Australian unit trust; or
 (ii) a holding entity of an Australian unit trust.
 (4) For the purposes of subsection (2), the entity must be:
 (a) for an acquisition of interests in shares or an issue of shares:
 (i) a corporation that is a relevant entity that carries on an Australian business, whether alone or together with one or more other persons; or
 (ii) the holding entity of such a corporation; or
 (b) for any other action relating to a corporation:
 (i) an Australian corporation that carries on an Australian business, whether alone or together with one or more other persons; or
 (ii) the holding entity (other than a foreign corporation) of such a corporation; or
 (c) for an action relating to a unit trust:
 (i) an Australian unit trust; or
 (ii) a holding entity of an Australian unit trust.
55E Meaning of reviewable national security action—businesses
 (1) An action is a reviewable national security action if:
 (a) the action is taken, or proposed to be taken, by a person and the action is any of the following:
 (i) to acquire an interest of any percentage in an Australian business;
 (ii) to acquire an interest in the assets of an Australian business;
 (iii) to enter or terminate a significant agreement with an Australian business; and
 (b) as a result of the action or proposed action:
 (i) a foreign person acquires, or will acquire, a direct interest in the Australian business and that acquisition, or proposed acquisition, is not a significant action, notifiable action or notifiable national security action; or
 (ii) a foreign person will be in a position, or more of a position, to influence or participate in the central management and control of the Australian business; or
 (iii) a foreign person will be in a position, or more of a position, to influence, participate in or determine the policy of the Australian business; and
 (c) the action is or was not otherwise a significant action, a notifiable action or a notifiable national security action.
 (2) An action is also a reviewable national security action if:
 (a) the action is taken, or proposed to be taken, by a foreign person and the action is to start an Australian business; and
 (b) the action is or was not otherwise a significant action, a notifiable action or a notifiable national security action.
[...]
66A National security review of actions
Review allowed if action may pose a national security concern
 (1) Subject to subsection (5), the Treasurer may review:
 (a) a reviewable national security action; or
 (b) a significant action that is not a notifiable action or notifiable national security action;
if the Treasurer considers that the action may pose a national security concern.
[...]
67 Order prohibiting proposed significant actions
 (1) The Treasurer may, by notifiable instrument, make an order under subsection (2) if the Treasurer is satisfied that:
 (a) a significant action is proposed to be taken; and
 (b) taking the significant action would be contrary to the national interest.
 (1A) The Treasurer may, by notifiable instrument, make an order under subsection (2) if the Treasurer is satisfied that:
 (a) an action of any of the following kinds is proposed to be taken:
 (i) a notifiable national security action that is not a significant action;
 (ii) an action in respect of which the Treasurer has given a notice under subsection 66A(4);
 (iii) a reviewable national security action notified to the Treasurer; and
 (b) taking the action would be contrary to national security.
 (2) The Treasurer may make an order in accordance with the following table.
Powers of Treasurer to prohibit proposed actions
Item | If the action is ... | the Treasurer may make an order prohibiting ...
1 | to acquire a direct interest in an Australian entity that is an agribusiness, or an interest in securities in an entity | the whole or a part of the proposed acquisition.
2 | to issue securities in an entity | the whole or a part of the proposed issue of the securities. [...]
5 to acquire a direct interest in an Australian business that is an agribusiness, or an interest in assets of an Australian business | the whole or a part of the proposed acquisition. [...]
8 | to acquire an interest in Australian land | the proposed acquisition.
9 | to take any significant action prescribed by regulations made for the purposes of section 44 | any thing prescribed by regulations made for the purposes of this item.
[...]
81 Compulsory notice of notifiable actions
 (1) A foreign person who proposes to take a notifiable action must give a notice to the Treasurer before taking the action.
 (2) If a person takes an action by entering an agreement, the notice must deal with each notifiable action covered by the agreement.[...]" (Foreign Acquisitions and Takeovers Act 1975 (Act No. 92 of 1975), last amended by Australia's Foreign Relations (State and Territory Arrangements) (Consequential Amendments) Act 2020 on 1 January 2021, Sections 4, 26, 40, 41, 44, 47, 51, 55B, 55D, 55E, 66A, 67 and 81)
----------------------
"8AA Meaning of national security business
 (1) A business is a national security business if:
 (a) the business is carried on wholly or partly in Australia whether or not in anticipation of profit or gain; and
 (b) it is publicly known, or could be known upon the making of reasonable inquiries, that the business is of a kind referred to in subsection (2).
 (2) A business is of a kind referred to in this subsection if:
 (a) the business is a responsible entity (within the meaning of the Security of Critical Infrastructure Act 2018) for an asset; or
 (b) the business is an entity that is a direct interest holder in relation to a critical infrastructure asset (within the meaning of those terms in the Security of Critical Infrastructure Act 2018); or
 (c) the business is a carrier or nominated carriage service provider to which the Telecommunications Act 1997 applies; or 
[...]
22 Businesses that are sensitive businesses
 (1) This section prescribes conditions for subsection 26(1) of the Act.
 (2) A business is a sensitive business if:
 (a) the business is carried on wholly or partly in any of the following sectors (including such a business relating to infrastructure for those sectors):
 (i) media;
 (ii) telecommunications;
 (iii) transport; or
[...]
51 Taking action in relation to entities and businesses 
 The following table prescribes values for section 51 of the Act (entities and businesses) to the extent that that section relates to items 2 to 5 of the table in that section. 
Threshold value for entities and businesses
Item | For action taken by ... | the value (in millions of dollars) is ...
1 | an agreement country or region investor where the action relates to: (a) an entity, or a subsidiary of an entity, that is not carrying on a  sensitive business or a trustee of a trust that does not hold assets of a sensitive business; or (b) a business that is not a sensitive business | 1,192
2 | any foreign person in any other circumstances | 275.[...]" (Foreign Acquisitions and Takeovers Regulation 2015 (SLI 2015 No. 217), last amended by Foreign Investment Reform (Protecting Australia's National Security) Regulations 2020 on 1 January 2021, Sections 8AA, 22 and 51)
Foreign Acquisitions and Takeovers Act 1975 (Act No. 92 of 1975), last amended by Australia's Foreign Relations (State and Territory Arrangements) (Consequential Amendments) Act 2020 on  1 January 2021, Sections 4, 26, 40, 41, 44, 47, 51, 55B, 55D, 55E, 66A, 67 and 81
Foreign Acquisitions and Takeovers Regulation 2015 (SLI 2015 No. 217), last amended by Foreign Investment Reform (Protecting Australia's National Security) Regulations 2020 on 1 January 2021, Sections 8AA, 22 and 51
[<a href="https://www.legislation.gov.au/Details/C2021C00022/Html/Text">Act</a>],[<a href="https://www.legislation.gov.au/Details/F2021C00062">Reg</a>],[<a href="https://firb.gov.au/guidance-notes">Guidance</a>],[<a href="https://firb.gov.au/sites/firb.gov.au/files/2021-01/Australias_foreign_investment_policy.pdf">Policy</a>]