A different policy by Governor Lachlan Macquarie, of a new penal colony every year instead of piling up convicts in Sydney would likely have led to convict settlement of New Zealand (and not incidentally saved Macquarie's job from criticism in Parliament that New South Wales was becoming too soft a colony to deter criminality). But that's a pre-1900 POD.
What would make a truly interesting POD would be if Lionel Curtis's 1937 idea of federation of the UK with it's predominantly white colonies partially catches on in 1940, leading to a federation of Australia, New Zealand and Canada in the face of fears of abandonment by both the UK and the US. Perhaps the capital of such a federation could be at Auckland or Wellington, which would make the best sense in terms of time zones. A federation of this nature could also be made to include the British West Indies with less fear of racial "imbalance" as well as nonwhite colonies such as Fiji and the Solomon Islands, and with 25 million people, would even initially, rival the UK in GNP. And at the time, perhaps Allied occupied Greenland and Iceland and Faeroes as well.
Yes, Australia and Canada are geographically distant from one another. But by WWII, with radio, airplanes, trans-pacific cables and ships that can travel from Vancouver to Auckland in 14 days (Sydney 16 days), the difference is much less than say, between Sacramento and Washington DC in 1850 when California joined the United States.
What would make such a TL TRULY interesting would be the way such a federation, growing to 50 million people by the 1980s could provide a true social democratic counterbalance and alternative to the United States as the US goes more conservative from the 1980s to the present. Or even in the 60s and 70s. Such a federation, particularly if it also included the West Indies with it's non-white populations (migrating to Australia and Canada) might be amenable to annexing West New Guinea when the Dutch wanted out of it, instead of letting Indonesia take it over. And might even threafter permit unrestricted migration of New Guineans to Australia and Canada leading perhaps to a plebiscite for statehood for New Guinea instead of decolonization and statehood without a vote of Papua-New Guineans
.( For make no mistake, and I fully realize that this will set the cat amongst the pidteons.. The decolonization and statehood of Papua-New Guinea without a plebiscite, as was handled IOTL was as much an act of Grand Apartheid as South Africa's internationally condemned and unrecognized belated attempt to do the same thing with it's bantustans. Australia, because it kept up the wall between PNG and the Solomons and Australia (and abollished Kanak slave labor in Queensland) avoided most petty apartheid (except for Aboriginals who made up less than1% of the population). Australia was thus able to make grand apartheid work in a way that South Africa did not (ironically even after White Australia Policy was abandoned in 1975) and be able to in effect, hypocritcally tell South Africans, do as I say, not as I, Australia , did).
Anyway, an Australia-Canada federation of then 50 million people might not feel a need to do this (and if the West Indies are part of the federation, might be politically constrained against doing this). And might permit Asian immigration sooner. And might even annex some British Indian Ocean colonies as Mauritius and the Seychelles and the Chagos and Maldives.
Such a federation would have made sense then. And it makes even more sense now IOTL.