The Federation of Australia was the process by which the six separate British self-governing colonies of Queensland, New South Wales, Victoria, Tasmania, South Australia, and Western Australia agreed to unite and form the Commonwealth of Australia, establishing a system of federalism in Australia. The colonies of Fiji and New Zealand were originally part of this process, but they decided not to join the federation. Following federation, the six colonies that united to form the Commonwealth of Australia as states kept the systems of government (and the bicameral legislatures) that they had developed as separate colonies, but they also agreed to have a federal government that was responsible for matters concerning the whole nation. When the Constitution of Australia came into force, on 1 January 1901, the colonies collectively became states of the Commonwealth of Australia.

The efforts to bring about federation in the mid-19th century were dogged by the lack of popular support for the movement. A number of conventions were held during the 1890s to develop a constitution for the Commonwealth. Sir Henry Parkes, Premier of the Colony of New South Wales, was instrumental in this process. Sir Edmund Barton, second only to Parkes in the length of his commitment to the federation cause, was the caretaker Prime Minister of Australia at the inaugural national election in 1901 in March 1901. The election returned Barton as prime minister, though without a majority.

This period has lent its name to an architectural style prevalent in Australia at that time, known as Federation architecture, or Federation style.

New South Wales (commonly abbreviated as NSW) is a state on the east coast of Australia. It borders Queensland to the north, Victoria to the south, and South Australia to the west. Its coast borders the Coral and Tasman Seas to the east. The Australian Capital Territory and Jervis Bay Territory are enclaves within the state. New South Wales' state capital is Sydney, which is also Australia's most populous city. In June 2020, the population of New South Wales was over 8.1 million, making it Australia's most populous state. Just under two-thirds of the state's population, 5.3 million, live in the Greater Sydney area. The demonym for inhabitants of New South Wales is New South Welshmen.The Colony of New South Wales was founded as a British penal colony in 1788. It originally comprised more than half of the Australian mainland with its western boundary set at 129th meridian east in 1825. The colony then also included the island territories of Van Diemen's Land, Lord Howe Island, and Norfolk Island. During the 19th century, most of the colony's area was detached to form separate British colonies that eventually became the various states and territories of Australia. However, the Swan River Colony was never administered as part of New South Wales.

Lord Howe Island remains part of New South Wales, while Norfolk Island has become a federal territory, as have the areas now known as the Australian Capital Territory and the Jervis Bay Territory.