The Constitution of Australia, is the supreme law under which the government of the Commonwealth of Australia operates, including its relationship to the States of Australia. It consists of several documents. The most important is the Constitution of the Commonwealth of Australia, which is referred to as the "Constitution" in the remainder of this article. The Constitution was approved in a series of referendums held over 1898–1900 by the people of the Australian colonies, and the approved draft was enacted as a section of the Commonwealth of Australia Constitution Act 1900 (Imp), an Act of the Parliament of the United Kingdom.

The Commonwealth of Australia Constitution Act 1900 (Imp) was given Royal Assent on 9 July 1900, was proclaimed on 17 September 1900, and entered into force on 1 January 1901. Even though the Constitution was originally given legal force by an Act of the United Kingdom parliament, the Australia Act 1986 removed the power of the United Kingdom parliament to change the Constitution as in force in Australia, and the Constitution can now only be changed in accordance with the prescribed referendum procedures in Section 128.

Other pieces of legislation have constitutional significance for Australia. These are the Statute of Westminster, as adopted by the Commonwealth in the Statute of Westminster Adoption Act 1942 (retroactive to 1939), and the Australia Act 1986, which was passed in equivalent forms by the United Kingdom Parliament and the Australian Federal Parliament (using legislative powers conferred by enabling acts passed by the Parliaments of every Australian state). The Statute of Westminster Adoption Act is often regarded as the point at which Australia became de jure an independent nation, while the Australia Act for all practical purposes severed the remaining constitutional links between Australia and the United Kingdom. Although the monarch of the United Kingdom remains also the monarch of Australia, today this person, currently Queen Elizabeth II, acts in a distinct capacity as monarch of each.

Authority to interpret the Constitution lies exclusively with federal courts: with the Federal Court of Australia and, finally, the High Court of Australia