This only reinforces my point that the condition of "New Zealandness" is first and foremost a legal concept, and that the way it affects people is through the coercive power the concept does and doesn't deploy.

I'd have to disagree on this.
Being a New Zealand Resident or Citizen are a legal concepts. However I know people who have emigrated overseas, and are now Citizens of other countries that have required them to give up their New Zealand Citizenship to become so, but are still considered a "New Zealander".
I also know people who are legally New Zealand Citizens here, but are not consider New Zealanders, by themselves or others around them. They're accepted, welcomed, even praised for becoming Citizens, but just not considered "New Zealanders".

There's something cultural that applies to be considered a "New Zealander", related to a sense of having grown up here.
I don't know the definition for it, or even if there is a precise definition. I can tell you I know that there's a sense that even some people who've grown up here aren't true "New Zealanders", notably people who've never been outside Auckland.