What it does
The Royal Style and Titles Act 1973 is an enabling statute whose sole substantive function is to record the assent of the Parliament of Australia to a proposal that the monarch adopt a revised form of royal style and titles for exclusive use "in relation to Australia and its Territories" (s 2(1)). The Act does not itself declare the new title; instead it authorises Her Majesty to issue a Royal Proclamation under the Great Seal (or such seal as she appoints by Warrant) that will bring the new style into effect once published in the Commonwealth of Australia Gazette (s 2(2)).
The substance of the change is set out in the Schedule, which prescribes the exact words: "Elizabeth the Second, by the Grace of God Queen of Australia and Her other Realms and Territories, Head of the Commonwealth." This formulation is expressly stated to be "in lieu of" the style previously adopted under the Royal Style and Titles Act 1953 (s 2(1)). The preamble supplies the constitutional and diplomatic context: it recites that the 1953 Act had implemented a style settled by Proclamation on 28 May 1953, that the Australian Government now "considers it desirable to propose to Her Majesty a change", and that the new form "retains the common element referred to in the preamble to the Royal Style and Titles Act 1953".
In practical legal operation the Act therefore performs three linked tasks. First, it satisfies any constitutional convention or legislative precondition requiring parliamentary consent before the monarch's titulature can be altered for Australian purposes. Second, it supplies the domestic legal footing for the subsequent Proclamation. Third, it ensures that, from the date of Gazette publication, all Australian official usage—statutes, proclamations, writs, coinage, oaths, and diplomatic instruments—must deploy the new style. The Act is not ambulatory; once the Proclamation has issued and been gazetted the statute has exhausted its function, although it remains on the statute book as the formal record of parliamentary approval.