Interesting. New Zealand seems like an odd choice for a prepper bunker. I’m sure it is wonderful to live there now, but that seems irrelevant if civilization collapsed. I have been working under the assumption that there were billionaire bunkers littering the Rockies and Appalachians.
Personally, if I was a billionaire, I would go full Fallout and build several sustainable vaults of various kinds that could each hold enough people to rebuild the human race from. That and dump every spare dollar possible into an off-world colony.