INZ send cases to whichever office is most suitable at the time. This may be because at times, certain offices are specializing in a particular kind of application, or at other times one office has had a very high number of submissions, and therefore cases are sent to different offices with a lighter workload just then.
The Northern Region Documentation Branch is based in Manukau, and routinely deals with paperwork for all the Auckland area offices. That's not to say that when your case is assigned, it will be worked there. It might be worked locally, or not, but all offices apply the same regulations, so there's nothing to worry about.