In 1890, talks between the states on a federation of Australian (or rather Australasian) colonies began. Among the colonies at these talks was New Zealand. There was serious talk of New Zealand becoming a state of Australia, but it never came to be. Why was this?
One of the reasons why New Zealand never became part of Australia was that the leaders of New Zealand were deeply concerned about the place Maoris would have in Australia, taking into account the hardships faced by Aboriginal Australians who didn't even have citizenship, let alone the right to vote. Unlike their brother nation, New Zealand had a long history with their Maori inhabitants, often consisting of wars and land deals. In the NZ Parliament, they had four seats saved, specifically, for Maori representatives, which was better than the zero representation that Aboriginal Australians got.
In Delegate William Russel's own words, they had no interest in ceding power to a government which cared nothing for native administration and dealt with natives in a more summary manner than the New Zealanders did.
Australia, in an attempt to sway New Zealand to join, gave Māori the right to vote in 1902, while Australian Aborigines could not vote until 1967. Of course, New Zealand was still not interested.
Another issue was that Australia's proposed system of government was to include plural voting, which meant landowners would get additional votes, a system which New Zealand had just abolished in 1889. Of course, Australia never went ahead with plural voting, but it was still a turn-off for representatives from New Zealand.
But what if New Zealand and Australia were able to resolve, or at least are able to reach a compromise on these issues, and New Zealand agreed to become part of Australia in 1890, eventually formally uniting with it under Federation in 1901?