An effective way of protecting national assets is to control access to them.
National security access controls include a system of identifying sensitive material and premises with classifications such as CONFIDENTIAL, SECRET and TOP SECRET. Only people with appropriate security clearances are allowed access to such premises or material. Before a government agency grants someone this sort of clearance, it will check the person's general suitability and ask ASIO for a 'security assessment' of their background and any past activities which may indicate they could be a threat to national security.
People who are dissatisfied with the results of a security assessment provided by ASIO have the right to ask for a review by the Security Appeals Division of the Administrative Appeals Tribunal.
Another way of protecting sensitive material and areas, and of protecting Australia's way of life, is to prevent certain people from entering or staying in Australia. Part of ASIO's protective security work is to assess whether people applying for entry or permanent residence visas have the potential for espionage, have links with a terrorist organisation, or may in other ways be a threat to national security. ASIO then advises client government departments (normally the Department of Immigration and Citizenship or the Department of Foreign Affairs and Trade) whether the person poses a risk to Australia's security.