On this day December 11, 1931
The British Parliament enacted the Statute of Westminster, giving the option of complete legislative independence to the Irish Free State, Newfoundland, Canada, New Zealand, Australia and South Africa.
The Statute is an Act of the Parliament of the United Kingdom which established a status of legislative equality between the self-governing dominions of the British Empire and the United Kingdom, with a few residual exceptions.
The Statute remains domestic law within each of the other Commonwealth realms, to the extent that it was not rendered obsolete by the process of constitutional patriation.
The Statute is of historical importance because it marked the effective legislative independence of these countries, either immediately or upon ratification.
The residual constitutional powers retained by the Westminster parliament have now largely been superseded by subsequent legislation. Its current relevance is that it sets the basis for the continuing relationship between the Commonwealth realms and the Crown.
The Statute applied to the dominions which existed in 1931: the Commonwealth of Australia, Dominion of Canada, the Irish Free State, the Dominion of Newfoundland, the Dominion of New Zealand, and the Union of South Africa. It excluded revisions of the Acts of Parliament upon which the constitutions of Canada and Australia were founded. New Zealand’s constitution is unwritten.
Further, it did not apply to Australia, New Zealand or Newfoundland unless and until ratified by their respective Parliaments. Australia ratified the Statute in 1942; to clarify government war powers, the adoption was backdated to September 3, 1939—the start of World War II. New Zealand adopted the Statute on November 25, 1947 by its Statute of Westminster Adoption Act. Newfoundland never adopted the Statute; by request of its government, the United Kingdom resumed direct rule in 1934 and maintained it until Newfoundland became a province of Canada in 1949.
Official text of the statute as amended and in force today within the United Kingdom, from the UK Statute Law Database