Well for one will not be considered a western nation, literally there are western nations that have more números native population or outrigth mayorities that, because of that fact, aren't considered western nations.
Like México. Is not a western Nation? I mean is literally USA neighbor, with a big native and mestizo population, important industrial capacity, bigger than anything New Zealand have, member of the OCD, fight with the allies during the WWII since 1942, and somehow is not considered a western Nation.
Probably the same will the fate of New Zealand if they were a majority native population.
That being said New Zealand is not the higesth western nation on percentage of native population. Most of the non anglo América have a similar or greater percentage than New Zealand, notable exception being Argentina Uruguay and Brazil
And we even can discuss if New Zealand is a western nation
The challenge isn't asking how to make a Maori majority New Zealand remain a "Western" country, though. The OP is just asking how to make New Zealand majority indigenous, while also pointing out that New Zealand in OTL is a Western country with a proportionally high indigenous population. I interpret that to mean it doesn't matter if the alternate New Zealand is Western or not.
A lot of people in this thread seem to be interpretting "Western" to mean the UK and its former settler colonies. I've never heard this definition. "Western" typically means the countries of Western European and countries founded and settled by people of Western European cultural origins. This means not only the UK and English-speaking of North America and Oceania, but also countries like France, Sweden, Spain, Germany, Italy, etc.
The common features are a shared history of inheriting Greco-Roman knowledge and philosophy, adopting Christianity, undergoing the Enlightenment, and benefiting from colonialism, among other minor features.
It's not an easy thing to define. Huntington left Eastern Europe out of it on account of the Great Schism and communism, for example. That makes the UK and France clear examples of Western countries and Russia a clearly non-Western country, but it gets murky in the middle. Poland, Czech Republic, and Slovakia - Slavic-speaking like Russia but use the Latin alphabet rather than Cyrillic, adopted Western Christiandom as opposed to Orthodox, and went through a period of time under the Iron Curtain before opening up to join the EU and NATO. Greece - Most people claim Greece is the origin of Western Civilization but is modern Greece a "Western" countey? Historically, it evolved in a very different way from Western Europe, with its own alphabet, an Orthodox religion, and a very different culture.
When you get outside of Europe, it gets murkier. If the US, Canada, Australia, and New Zealand are Western countries, what about Latin America? The system of government and the culture of the elite are obviously Western, but what about the culture of the common people of Mexico, Guatemala, Peru, or Bolivia, which are just as much influenced by their ancestral indigenous cultures as they are influenced by the West. Many Latin American and Caribbean countries have a strong African influence as well. Does that make Argentina and Uruguay Western? What about Chile? Is Bolivia a country transitioning from Western back to non-Western as the indigenous majority becomes powerful again? What about India and African countries which have thoroughly Western systems of government and Western languages as the lingua franca but whose majorities maintain cultures much older than European colonialism, or Pacific Island nations which have been heavily Christianized?
Ultimately the definition of "Western" is too hard to narrow down in concrete terms. It also works when contrasted with countries that are clearly non-Western like China or Iran or Vietnam or Saudi Arabia.
As for the OP, I suppose the key is to prevent European settlement until the Maori have a chance to recover from the imported disease epidemics. If the Maori are more hostile, it will be more difficult for the Europeans to settle down and maybe they'll give up.