There's some very decent ethnological evidence that the Maori practiced cannibalism; so being taken a prisoner in the endless intergroup fighting of pre/pericontact Maori culture was not exactly an augury of a brilliant future.

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This is kind of an understatement. From what I've read, its quite common for Maori in the common day to trash talk other iwi by saying things like...
"Their not so tough, we were still eating them 100 years ago."
There was an interesting ecological effect as a result: when a Maori group faced defeat in battle, they basically decamped before they could get slaughtered/eaten. This wasn't easy, because precontact Maori populations were pretty dense, so all the good places were already taken.
According to Jared Diamond, one of the only reasons the british were able to establish a foothold at all on NZ was the fact that their germs thinned out the population substantially. Abel Tasman, the discoverer of NZ, was driven off by a band of Maori.
There was one way out, though—the fire escape, so to speak, but it was rough. Basically, you bolted for the southern island rainforests and set up shop there anew... surrounded by trees some of which had twelve to fourteen foot diameters and crowded together as thick as blades of grass in a well-tended lawn. You had to figure out how to cut these enormous trees down, uproot their tough-as-steel root systems, and carry out agriculture under pretty inhospitable conditions—rain is nice, but several hundred inches of rain a year, in the rainforest ecosystem? Not nice at all!
Another one of my stops in August will be in Westland. Hopefully, some of these ecosystems still exist. It would be awesome to see that! I wouldn't want to try and eek out a living though...
This endless warfare cycle was therefore the main reason that the population of the southern island was so relatively dense at the time the Euros showed up.
Just to add another dimension to this, the period of time that preceded the arrival of most Europeans is known as the Musket Wars. During pre european times, iwi warfare was very intense, but most tribes were evenly balanced so not alot of social upheaval occured. Still, competition was intense and the amount of stress the tribes exerted on one another was considerable.
When the first Europeans arrived, those that were in closest contact with said europeans traded everything they had for...you guessed it...muskets. In fact, maori cheifs who were brought to England and showered with gifts, sold those gifts and bought muskets for their iwi. This caused a great imbalance of power between the iwi as the few who had muskets proceeded to slaughter those who did not.
The histories of the musket wars are wraught with stories of 1000 human barbecues. The europeans stood on the outside and watched as Maori society self destructed.
This is how the islands initially became depopulated. Disease came next. And STILL the british could not conquer all of NZ.
Maori are badass.
In the typical cycle-of-conquest scenarios that gave rise to the great early empire/civilisations of Egypt, the Indus Valley, Mesopotamia and the South American highlands, you got conquered, you got to be slaves, very unpleasant but doable. In aboriginal NZ, you got conquered, you got eaten. Not doable at all...
Agreed.