I would assume most of their employees in NZ hold NZ citizenship although I'm unsure exactly how NZ handles ITAR.
They have engineers/technicians based in the US as well and I would assume most of those are US Persons (US Citizens or Permanent Residents). In the US you can get ITAR exemptions for non-US Persons but from my understanding it's a more difficult and much more expensive process so it is only done to recruit very high level talent.
I'm at SpaceX, which is a much larger company than Rocket Lab, and I can probably count on two hands the number of non-US Persons working here.
I'm fairly certain that Peter Beck, Rocket Lab's founder and CEO, who is a Kiwi, lives in Socal now.