Who made the decision for federation to happen and how long did the process take?
Before 1901, Australia was not a nation. At that time, the continent consisted of 6 British colonies—New South Wales, Victoria, Queensland, South Australia, Tasmania and Western Australia—which were partly self-governing, but subject to the law-making power of the British Parliament.
During the 1890s, each colony sent representatives to special meetings, called conventions, to try to agree about how to form a new Federation. Eventually the delegates agreed on the rules for a national parliament and a draft constitution. The people of the colonies voted in a series of referendums to accept this new Australian Constitution. It was then passed as an Act of the British Parliament in 1900, called the Commonwealth of Australia Constitution Act 1900, which came into effect on 1 January 1901. The Constitution established an Australian Parliament which could make laws on behalf of the new Australian nation.
The colonies of New South Wales, Victoria, Queensland, South Australia and Tasmania united and became the states of Australia. Western Australia was not a party to the initial agreement but agreed to join the federation before 1 January 1901.