I did 6 weeks in NZ last year, both islands. If I only had a week, I'd consider a short trip north of Auckland, to Paihia on the Bay of Islands. Further north, so warmer. They have some good historical locations etc. Or, of course, Rotorua. Hot springs and Maori. Maybe the Coromandal Pensula (I didn't get there so it's on my short list for next time) But, just Auckland and one other spot.