To be honest, not knowing much about the adoption process I couldn't elaborate much more.

Going by the normal rules of family sponsorship (which would be the case if you moved abroad and your natural child wanted to join you at a later date) you have to be in New Zealand for 5 years before being able to sponsor family members. If the child wasnt included on the original application for residency (which I'm assuming they wouldnt be, depending if you know the child you wish to adopt yet) then NZIS want to know why they weren't originally included, and why you are trying to sponsor them now.

The rules for adopting form outside of New Zealand may well be completely different and I cannot find any guidelines on this subject.

I know that for slightly different circumstances, people who marry someone not from New Zealand have to provide a great deal of evidence to show that the relationship is a genuine and stable one, and not just a means of illegal immigration.

You really are best off speaking directly with immigration on this one. Its a bit too complicated for me!