After 1840 when the treaty of Waitangi was signed and NZ became a colony of Britain. Is there a conceivable way of someone else taking over, maybe with rebel Maori help or some such. I just can't really see it happening without there being or being from major butterflies removing most of British Imperial power. Is it do-able?
Maybe. I think the most realistic POD would be some sort of Great Power diplomatic arrangement. Where NZ is traded or given away for some other, more important reasons. Whatever that might be.
There is a narrow window of opportunity here, as NZ was not heavily settled by the British till the early 1850s, after which point things started massively skewing towards the Settlers, demographically. Responsible government being granted/enacted around 52-54.
The British in NZ up to that point were much less solidly placed - lots of traders based out of the Australian colonies, missionaries, freed convicts looking for a new start. They numbered in the low thousands (under 2?) by 1840 and not a lot more by 1850. By 1853 or so they were getting close to parity with the Maori.
The 1840s saw a few planned settlements, mainly in the North Island and northern South Island. The planned settlements in the sourthern/central South Island, where my ancestors came into the picture, didn't occur till the 1850s.
The Australian colonies would be very annoyed if this happened, as they saw NZ as their economic hinterland. Which it remains till this day. There would be a lot of people in the UK proper who would be annoyed too, as the project to make NZ British enjoyed widespread public support.
But, end of the day, it was a marginal colony and could be relatively easily traded away up till this point. So, find a reason why it would be!