On 9 July 1900, Queen Victoria gave royal assent to the Commonwealth of Australia Constitution Act. The Act had been passed by the British Parliament and gave legal force to the federation of the Australian colonies.

Federation had not been created in London. Australian politicians and voters had debated the terms of union for years, and the colonies had held referendums on the proposed Constitution. The result was a plan for a federal Commonwealth, with powers divided between a national parliament and the former colonies, which would become states.

But the colonies were still part of the British Empire. To create the new Commonwealth, the Constitution had to be enacted through Westminster. Australian delegates came to London wanting the agreed text passed with as little alteration as possible. British ministers examined the Bill because it affected imperial authority and appeals to the Privy Council.

Royal assent on 9 July made the Act law, but it did not bring the Commonwealth into existence that day. The Act allowed Queen Victoria to set the starting date by proclamation. That date was later fixed as 1 January 1901.

The new Commonwealth of Australia remained within the British Empire. The monarch continued to have a constitutional role, and links with Britain remained part of the new system. Even so, the Act created a national Australian government and marked a major transfer from separate colonial governments to federation.

The royal assent of 9 July 1900 was a British constitutional act with Australian consequences. It made possible the Commonwealth that began six months later.