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.
"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. [...]
substantial interest: a person holds a substantial interest in an entity or trust if:
(a) for an entity—the person holds an interest of at least 20% in the entity; 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).
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).
[...]
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 an Australian entity or 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).
Third condition—kinds of entities covered
(4) If paragraph (2)(a) or (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. [See source for table]
[...]
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.
(2) The Treasurer may make an order in accordance with the following table. [See source for table]
(3) If the Treasurer makes an order under subsection (2), the Treasurer may, by notifiable instrument, also make an order in accordance with the following table. [See source for table]
[...]
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 18 September 2018, Sections 4, 26, 40, 41, 44, 47, 48, 51, 67, and 81)
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"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 | 1,094
2 | any foreign person in any other circumstances | 252.[...]" (Foreign Acquisitions and Takeovers Regulation 2015 (SLI 2015 No. 217), last amended 30 December 2018, Sections 22 and 51)
Foreign Acquisitions and Takeovers Act 1975 (Act No. 92 of 1975), last amended 18 September 2018, Sections 4, 26, 40, 41, 44, 47, 48, 51, 67, and 81
Foreign Acquisitions and Takeovers Regulation 2015 (SLI 2015 No. 217), last amended 30 December 2018, Sections 22 and 51
[<a href="https://www.legislation.gov.au/C2004A01402/2018-09-18/text">Act</a>],[<a href="https://www.legislation.gov.au/F2015L01854/2018-12-30/text">Reg</a>]