I don't get why they'd think New Zealand would be safe from protests. If they really want to be safe they should be thinking places like, say, Heard Island, like a true billionaire supervillain would.
You're looking at it the wrong way :)
Heard Island would be good place for a single loner supervillian billionaire with enough loyal henchmen and minions, but not so good for the ones that want a bit more room and to easily visit with the other billionaires to race their superyachts. They don't want to get too lonely.
New Zealand is good for them because it is still hard to reach by poor boat people, and easyish to defend from the ones that can make it that far. Especially with some modest defense investment.
The local population isn't that high currently, so get enough billionaires there and it will be quite cost effective for them collectively to bribe/placate the locals into semi happiness.
They'll be able to double NZs GDP by handing out whatever they find behind the couch cushions. And they'll look like nice friendly philanthropists too.