Has New Zealand ever been part of Australia?
Australia, as a political entity was formed by the merger of six British colonies in 1901. The people who negotiated this merger proposed that New Zealand and Fiji should also be part of it, but the governments of New Zealand and Fiji rejected the proposal on the grounds of cultural difference, so they did not become part of it.
When the colony of New South Wales (which became the Australian state of New South Wales) was established, they defined it as including islands in the Pacific to the east of Australia, which meant that by definition it included New Zealand, Fiji and Rekohu (now known as the Chatham Islands), plus maybe some other places. However, New Zealand and Rekohu continued to be under the control of their native people (I'm not sure about Fiji). In the 1830s, New South Wales sent a governor to New Zealand to try to gain actual control off New Zealand. There he negotiated with Maori chiefs, and Englsh and French missionaries who were living among the Maori people, and they ended up signing a treaty called the Treaty of Waitangi. The Treaty of Waitangi possibly made New Zealand part of the British Empire, or possibly just gave the British Empire control over non-Maori settlements in New Zealand (depending on whose interpretation you go with). Either way, the Colony of New South Wales then became responsible for the British settlements in New Zealand (they didn't have the power to have practical control over Maori communities at the time).
Then 1 1/2 years later, in 1841, New Zealand was declared a separate colony.
TL/DR: New Zealand was effectively part of New South wales for 1 1/2 years, before there was a political entity called Australia for New South Wales to be part of.
New Zealand is mentioned as a state in the Commonwealth of Australia Constitution Act:
"The States shall mean such of the colonies of New South Wales, New Zealand, Queensland, Tasmania, Victoria, Western Australia, and South Australia, including the northern territory of South Australia, as for the time being are parts of the Commonwealth, and such colonies or territories as may be admitted into or established by the Commonwealth as States; and each of such parts of the Commonwealth shall be called a State. [emphasis added]."
The reason why New Zealand is not part of Australia, is twofold:
- Australia did not form from the New South Wales colony directly. From 1825–1890, various parts of New South Wales applied to be their own colonies, and 5: Van Dieman's Land (later called ‘Tasmania'), Queensland, Victoria, South Australia, Western Australia and New Zealand were granted their separation from New South Wales through Acts of the British Parliament.
- New Zealand was invited to the Constitutional Convention that re-merged all of these colonies into one country as independent states, but did not show up.
Because Australia was formed by various colonies coming together consensually, not by New South Wales becoming Australia, New Zealand got to decide whether or not it wanted to be a part of Australia. All of the states but New Zealand chose to be a part of Australia.