New Zealand had a special environment. From the time it split from Antarctica about 80 million years ago, it was one of the larger, more isolated land masses in the world*. The fact that it was mostly free of terrestrian mammalian predators** eems to have, of itself, fostered a unique panoply of ground-based wildlife.
A recent article in American Scientist (The Rat's Tale) paints a picture of ecological destruction that followed the first wave of predatory mammals - although the second wave (Europeans) would ultimately have been more destructive.
The article details the laborious scientific process - and debate - in establishing the date for that first wave, which last year was settled with a period range of 1290 to 1380AD for the arrival of the Maori and the Polynesian rat, kiore or Rattus exulans.
The rats ate "plants, fruits, seeds, insects, lizards, snails, eggs, and the nestlings of ground-breeding birds".
Maoris were apparently responsible for the extinction of the largely predator-free [ostrich-like] moa, and the reduction in forestation of New Zealand from about 85-90% to 25-27% (according to the article). Good evidence suggests this latter was due to significant burn-offs - for various reasons, including living space, ease of travel, and fostering food sources, specifically the bracken root, a starchy staple.
From that first wave, human and rat between them precipitated a 50% decline in bird species and the extinction of bat, frog, and numerous lizard species.
The oldest verified archeological site in NZ is Wairau Bar in the South Island, dating to 1285 to 1300. Amongst many artifacts are the bones of 8000-odd moas and 2000 moa eggs - suggestive of the ease with which they were killed.
The Polynesian rats apparently took less than 80 years to spread throughout the North and South Islands - about the same time as it took the European rat, Rattus norvegicus, once introduced.
This is not a tale of Maori destruction - rather, it's the familiar one of human destruction: "The first duty of colonisers is to survive. That requires rapid population growth sustained by consuming the richest resources" (Atholl Anderson, Australian National University).
The main reference for the article is: Wilmshurst, J. M., A. J. Anderson, T. F. G. Higham and T. H. Worthy. 2008. Dating the late prehistoric dispersal of Polynesians to New Zealand using the commensal Pacific rat. Proceedings of the National Academy of Sciences 105:7676–7680.
*Albeit some theories (discussed earlier, here) posit complete submergence of the land mass for a period of time. I'm not convinced of this, particularly given the continued existence of the flightless ratite moa, discussed here.
**The recently discovered 'waddling mouse'/'sb mammal' -see here and here - notwithstanding. There doesn't seem to be any evidence that it impacted the significant and burgeoning terrestrial habiting bird population.