Factors:
1. Radioactive fallout patterns in a hot nuclear war between any of US, China, Russia, Europe. NZ has low target priority and wind driven fallout spread may be lower.
2. Spillover effects from climate-change driven mass human refugee migration (water and distance are an effective barrier from hundreds of millions of displaced people from Africa, South Asia, or Latin America who may be forced to flee areas that cannot grow food or which reach the wet bulb mark in temp+humidity).
3. Low population density relative to natural resources / nature / arable land.
4. Relatively high amounts of freshwater resources relative to population (and possible population post-refugee/migrants in an adverse war/climate scenario).
In the event that the proletariat rose up and demanded support from those in bunkers, the number of 'normies'/refugees that 'robber barons fled to NZ' would have to support is much lower than the number of revolutionary poor that could storm the Bastille in most other continents. The amount of 'bread and circuses' you need to supply to avoid ending like Versaille is cheaper. If you decide mercilessly to just kill any refugees / migrants in a post-apocalyptic scenario, you can do so while they are still in the water / on improvised rafts rather than defending an area on land.
Of course I may be wrong and am happy to be corrected on any of these points.