Since 4 April 2002, there are only a few circumstances in which Australian citizenship can be lost. However, prior to that date, thousands of Australian citizens lost their citizenship when they (or their parents, in some cases) acquired another citizenship.
Loss of Australian citizenship in these situations was automatic. It doesn't matter if the Australians found out about it or not. Many such people still carry Australian passports - these passports are invalid.
 Deprivation of Australian citizenship on character grounds
- A person who is an Australian citizen by conferral (also known as grant or naturalisation) can be deprived of Australian citizenship if:
- sentenced to 12 or more months imprisonment for an offence committed before the person became an Australian citizen; or
- convicted of "migration related fraud" relating to an application for citizenship or permanent residence
- Fairly unusual, but not impossible
 Adults acquiring another citizenship
- An adult Australian who acquired another citizenship between 26 January 1949 and 3 April 2002 as a result of a voluntary and formal act generally lost Australian citizenship, automatically.
- This does not include acquiring the passport of a country of which one was already a citizen.
- "Adult" is generally a person aged 18 or over, since 1 December 1973. Prior to that date, an "adult" was generally someone aged 21 or over. A married person may be considered an adult.
 Long residence outside Australia
- Prior to 8 October 1958, those Australian citizens by naturalisation or registration could lose Australian citizenship if outside Australia (or New Guinea) for a continuous period of 7 years, under Section 20 of the 1948 Act.
- Those who registered an intention to retain Australian citizenship were exempt;
- Those who left Australia after 8 October 1951 are not affected by this provision.
 Loss of Australian citizenship - children
- Children could not lose Australian citizenship independently, but could lose it based on the loss of a parent
 From 26 January 1949 to 21 November 1984
- As a general rule, an Australian child lost Australian citizenship under Section 23 of the Australian Citizenship Act 1948 if a. the responsible parent lost Australian citizenship under Sections 17 or 20 of the Act and b. the child also possessed another citizenship.
- In this time, the "responsible parent" was generally the father only. Unless the father was deceased, or separated/divorced, in which case it may have been the mother.
 From 22 November 1984 to 3 April 2002
- An Australian child could have lost Australian citizenship if a responsible parent of the child lost Australian citizenship under Section 17 of the Act (or renounced Australian citizenship) unless either of the following applied:
- the child did not possess any other citizenship; or
- the other responsible parent of the child remained an Australian citizen (or was an Australian citizen at death).
 From 4 April 2002 to 30 June 2007
- Section 17 of the Act was repealed, so children could not lose Australian citizenship based on a parent becoming a citizen of another country;
- However if a parent renounced Australian citizenship, or was deprived of citizenship, children could still automatically lose Australian citizenship.
 From 1 July 2007
- The Minister has discretion to revoke the Australian citizenship of children of a parent renouncing or being deprived of Australian citizenship, unless the other parent is an Australian citizens.
- Similar to the pre-2007 procedures, but no longer automatic.
 What happens when you lose Australian citizenship
- If you're in Australia, you automatically get an ex-citizen visa.
- This is a permanent visa, but without a re-entry facility. You need a Resident Return Visa to travel.
- An ex-citizen visa can be cancelled on character grounds.
 I lost my Australian citizenship, can I get it back?
- Usually yes, provided you are of "good character"
- If you lost Australian citizenship under Section 17 of the Act (but not Section 20 or 23) your children, born after you ceased to be Australian, can apply for a special grant of Australian citizenship.
 British nationality - prior to 26 January 1949
- Prior to 26 January 1949, Australians were British nationals
- An adult Australian who became a citizen of a foreign (non-British Commonwealth) country - eg the USA, Argentina or France - prior to 26 January 1949 automatically lost British nationality and hence never became an Australian citizen;
- It is noteworthy that married women (unless divorced/widowed) were legally under a "disability" and could not lose British nationality by their own actions. Hence, usually they did become Australian citizens on 26 January 1949, even if they had previously become an American citizen.