>I think New Zealand could do with a larger population in the longer term

Why?

Don't you think NZ is great because it has a small population? Our big cities are practically land locked. and greater populations just mean increased density, more pollution, and reduced quality of living. The 1/4 acre dream is already dead.
In addition because our major export is dairy, that is a land restricted activity which won't increase with population. As a result an increased population means we all get poorer. Dairy farming is becoming more intensive and our land is becoming slowly polluted by fertiliser because the production required to support our lifestyle at current population levels is already very high.
Secondly as AI gets better this century and production becomes ever more automated, the need for people only reduces.

So I think the opposite is true. We need to reduce our population to improve our lifestyle.