I lived on the north island for about 3 years and climbed heaps there.
Wharepappa (pronounced far-a-pappa, sort of) is great. Yeah the routes are mostly under 20 meters, but heaps of fun, and real good quality sport climbing (imho).
I'll second Whaganui Bay - very wicked. It is maori land, and the deal is you go to visit Mary on arrival to say hi and pay for camping (I think it is like 2 bucks). The road in can be a handful in the wet. Spectaular locatation and great climbing. There is a 'pub' there that you may be able to find .. and if you are lucky some locals may give you a fresh fish or two for dinner.. if you're not maori tho you are not allowed to fish - FYI.
It is a very special place.
One more place I reccomend even more is Te Ananui - on the coromanel peninsula. Have to go at low tide, but this place is AWESOME! One of the nicest locations I have climbed at (its on a beach, surrounded by cliffs).. Some good trad climbing here, be wary of the rock quality tho.
I was over there about a month ago - and some climbing mates lead me to believe there were some new 'secret' areas being opened up. Around mangetopop (sp?) area - Central north island.. if you go to the indoor Gym (Extreme edge) on a wed or thurs night you will find people to grill for beta..