I've never had any problem getting NZ Immigration to stamp my passport on arrival when asked (and I always ask!)
The stamp I've got doesn't say anything all that interesting, it just looks like :
NEW ZEALAND CUSTOMS SERVICE
01 JAN 2099
AUCKLAND AIRPORT
The date corresponds to the arrival date. I don't seem to have any departure stamps - not sure if that's because I didn't ask, or if it's because they said no.