Australian Citizenship for a person born in Australia. 1949 and 1986 Regulations
Australian Citizenship for a person born before 26 January 1949.
In most circumstances if a British subject was born in Australia before 26 January 1949, they automatically became an Australian citizen on 26 January 1949, unless their father was a foreign diplomat.
Australian Citizenship for a person born between 26 January 1949 and 19 August 1986.
In most circumstances if a person was born in Australia between 26 January 1949 and 19 August 1986, they are Australian citizens by birth unless their father was a foreign diplomat.
Australian Citizenship for a person born on or after 20 August 1986.
A person born in Australia on or after 20 August 1986, became an Australian citizen at birth if at least one of the parents was an Australian citizen or permanent resident of Australia.
A person born in Australia on or after 20 August 1986, became an Australian citizen at on their 10th Birthday if they had been ordinarily resident in Australia for the 10 years since birth, but neither of their parents were Australian citizens or permanent residents at the time of their birth.
Sources:
http://guides.dss.gov.au/guide-social-security-law/2/2/4/10 – Version 1.241 – Released 5 February 2018
Australian Citizenship Act 1948 – https://www.legislation.gov.au/Details/C2006C00317
Australian Citizenship Act 1973 – https://www.legislation.gov.au/Details/C2005C00625
Australian Citizenship Amendment Act 1984 – https://www.legislation.gov.au/Details/C2004A02993
Australian Citizenship (Transitionals and Consequentials) Act 2007 – https://www.legislation.gov.au/Details/C2007A00021