It would be extremely difficult to have a white minority-ruled state al a South Africa in New Zealand. First of all, the Maori were seen as a type of 'noble savage', and were often called the 'handsome natives'. They weren't seen as alien as Negros in Africa. The Maori had built a relatively advanced communal culture, far more so than the other Polynesian peoples, including the Hawaiians.
It's possible to give New Zealand, or at least the North Island, a Maori majority. What is difficult, though, is keeping the South Island as majority Maori. The South Island is very attractive to British settlers when gold is found, and the Canterbury plain is good for agriculture. Christchurch is the main city there, and its the second largest in New Zealand, after only Auckland. The only sizeable tribe there is Ngai Tahu, which controls lots of land, but has a relatively small population.
Julius Vogel, what do you mean by 'NI' tribes?
The Maori population experienced a boom after the introduction of potatoes (at the time, kumara/sweet potato were about the size of a thumb), due to the larger number of available calories. If the potato is introduced by the first settlers, we can see the Maori trading with them (almost all colonies were dependent on trade with the Maori) for potatos. I imagine that growing potatoes would be similar to growing kumara, so the Maori would already have a high level of expertise.
It is questionable whether the Nga Puhi, the most Northern, populous and warlike tribe, signed the Treaty of Waitangi (Te Tiriti o Waitangi) and they tended to have a fierce independent streak. Perhaps an analogue to the Musket Wars takes place, with the Nga Puhi winning before many significant tribes can get European weapons? A more centralised, powerful Maori authority would likely be able to weaken direct British control, even if de jure being British-controlled.
BTW, the Maori were affected by foreign diseases, but it seems not to the extend of Native Americans (which seems bizarre, Polynesians had no contact with any non-tropical diseases)
"Is there any way to get low-density settlement - for instance, most of NZ divided into a few huge sheep-raising estates, a few port towns but no real cities, and the Maori living everywhere that isn't being used to raise sheep? I'm thinking something like Punta Arenas in the late nineteenth century.
I assume the trick would be to have a few influential sheep barons stake claims to all the land before the yeoman farmers show up. Any idea who they might be, or if such a thing was even possible?" - Jonathan Edelstein
This sort of happened OTL. In many areas, absentee landlords took prime farming land, with Maori living elsewhere. And even now, in most places outside of the major urban centres, there are lots of people who classify themselves as "NZ European", when they clearly are of partial Maori descent.