When I was there a couple of years ago, I dove a couple of days at the Poor Knights, which is a must do I wish I had more time to spend there, and I also did a day around Kapiti Island. For Poor Knights I dove with Yukon Dive Charters (
Yukon Dive - Diving the Poor Knights, Tutukaka, Northland, New Zealand), Noel and Jo are awesome! I personally don't like large boats, and I was definitely happy with my choice over Dive Tutukaka when I saw how many divers they had on their boat (though I have heard they are a good op too, just not my cup of tea). Around Kapiti Island, I used Dive Kapiti (
http://divekapiti.co.nz/)out of Paraparumu. It was less touristy than Poor Knights, but I was happy with my dives, and would dive with them again. It was more like diving with the local guys than a vacation dive charter.
I hear the diving around Wellington can be nice as well, and I wish I would have had the time to do White Island which I hear is amazing, almost as good as Poor Knights, but that is a liveaboard trip as its quite far out. I don't know what your experience level is, but keep in mind that this is not warm water diving. I used a drysuit which I would highly recommend. Poor knights can be done in a 5mm if your not too temperature sensitive, 7mm if you want to be warmer. The further south you go the cooler the water will get. I don't have my dive log handy, but if you want to know the water temps on my dives, I'm sure I can look it up for you.
Enjoy your trip!
Mark