I've been mulling over this idea for a bit, but don't have much info on New Zealand, so I decided to ask for input. How plausible is a surviving Maori majority nation in New Zealand, or at least the north island?
It could happen, but but it would be unlikely.
I think the best way of doing it would be to introduce a 18th century POD - say an introduction of parts of the European food package. Maori proved pretty able to adapt to the crops introduced by the whalers/traders pre colonisation, to the extent of supplying the early Australian colonial markets in their own right.
So perhaps get one of the earlier explorers or traders introduce pigs, potatoes and other such more productive/colder weather crops. If that happens early enough then the Maori would be able to heavily colonise the entire country, not just the upper SI and the NI.
When British colonisation occurred the South Island was so underpopulated compared to the North that settlers were able to overwhelm the local communities very easily*. By comparison, it took a prolonged series of wars in the 1860s-70s to make the same true for most of the North Island. This gave the early colonial state a very secure base of operations.
Another POD would be to delay proper European colonisation of Australia's Eastern seaboard. If the British did not send the First Fleet and establish a colony then, or for a few decades, then the prospect of NZ being settled by British settlers is far less likely, as is the economic case for doing such.
* As an example, the area I grew up in had about three permanent Maori settlements (Otago), so far as I can remember from the last time I looked, with the permanent population being under 1000. Land theft/seizure, disease, war etc lowered that quickly and they were outnumbered by many times within a couple of years of the first British settlement in the 1840s