That’s easy: keep the guards’ families inside the bunker. Now who’s gonna switch sides? (see how it went with the Wagnerites: threaten families, stop a coup).
Also, New Zealand is not a coincidence, it’s far away from most of the mobs. With a local population of barely over 5 million, I bet they could hire some, kill some, help some a bit, and the rest would fall in line.
The thing I don’t get about building bunkers in NZ it’s…ummm… geologically active.
If you look at the South Island, the mountain range going from one end to the other is a big arse fault line just waiting for the next big shift. Following that up into the North Island from Wellington to Whakatane and Whakaari/White Island (you know the one that killed 22 people back in 2019) there’s the active volcanoes in the middle and the big lake that is a still active volcanic caldera.