So my question is how New Zealand's ecosystem is surviving without snakes?
Snakes are important in some ecosystems because they keep down vermin, but the natives of these places pay a high price for this service. Why does new Zealand not have any? Maybe like Ireland there was a water barrier preventing snakes from reaching the islands. Australia is swarming with snakes, most of them very poisonous, but maybe it wasn't when New Zealand broke free many millions of years ago.
Another possibility is that there were once a few snakes in New Zealand, but predators like the Giant Moa and its relatives, and snake-eating birds of prey, might have hunted them to extinction. As for Ireland, snakes were killed off or driven out by the Ice Age, and when the ice receded the snakes from Europe were too slow to get there before the Irish Sea was formed. Only three species manged to reach England before it was cut off by the Channel. Snakes are not important for British ecosystems, but not many people would like to see them become extinct here. Only one, the adder, is poisonous, and kills about one person every fifty years. Just as British ecosystems could survive quite happily without snakes, so can New Zealand. To introduce even harmless ones there could risk a similar eco-disaster to the one in Australia caused by the introduction of the cane toad.