Design and manufacturing for the rocket part of their business is mainly split between Auckland and Los Angeles. Various sources say they have between 500 and 700 employees in NZ.
My understanding is that they have a rocket body fabrication facility in NZ, where some of their historical institutional carbon fiber fabrication knowledge and stuff is, the engines and avionics and electronics are done in the Los Angeles area.
Earlier this year they acquired a new machining business in Auckland and are expanding capacity at their NZ launch site: https://rocketlabcorp.com/updates/rocket-lab-establishes-new...