The Flags Act 1953 is an act of the Parliament of Australia

Parliament of Australia

The Parliament of Australia, also known as the Commonwealth Parliament or Federal Parliament, is the legislative branch of the government of Australia. It is bicameral, largely modelled in the Westminster tradition, but with some influences from the United States Congress...

 which defines the official Flag of Australia

Flag of Australia

The flag of Australia is a defaced Blue Ensign: a blue field with the Union Flag in the canton , and a large white seven-pointed star known as the Commonwealth Star in the lower hoist quarter...

. Queen Elizabeth II

Elizabeth II of the United Kingdom

Elizabeth II is the constitutional monarch of 16 sovereign states known as the Commonwealth realms: the United Kingdom, Canada, Australia, New Zealand, Jamaica, Barbados, the Bahamas, Grenada, Papua New Guinea, the Solomon Islands, Tuvalu, Saint Lucia, Saint Vincent and the Grenadines, Belize,...

 gave Royal Assent

Royal Assent

The granting of royal assent refers to the method by which any constitutional monarch formally approves and promulgates an act of his or her nation's parliament, thus making it a law...

 on 14 February 1954 after opening the Commonwealth Parliament during her 1954 Royal Tour. It was the first of the few Commonwealth Statutes enacted by the reigning Monarch.
The legislation specifies the colours and construction details for the Australian National Flag and the Australian Red Ensign

Australian Red Ensign

The Australian Red Ensign resulted from the 1901 Commonwealth Government Federal Flag Design Competition which required two entries: a flag for official use and one for the mercantile marine...

 (also known as the Australian Merchant Flag).