Rob - it looks like you've already found the SDNZ forum (
www.scubadive.net.nz • Index page) which will be a mine of information!
There's so much good diving in NZ that you'll be running around like a dog chasing it's own tail if you're not careful! The "must dives" in my book are White Island (a live volcanco) and if you can convince the skipper to take you there, a dive at Volkner Rocks on the same day. Volkner Rocks is without a doubt one of the best dive sites in the world. Everyone goes on about how Jacques Cousteau put the Poor Knights in his top ten - but that's just because he didn't dive at Volkner.
Poor Knights are awesome, mind you. For techie days my charter of choice would be Yukon (
Poor Knights Islands Diving, Dive with Yukon). Noel is an awesome skipper, and a lot of the experienced techies here won't dive with anyone else by choice.
I'd also put Matai Bay Pinnacle as a top tech dive, the base of the pinnacle is about 45-50m and there's a beautiful black coral tree at the base. The pinnacle comes to just above the surface at low tide, so there's plenty of stuff to look at as you do your deco. A-Z diving in Whatuwhiwhi (
Learn to Dive with A to Z Diving - PADI school at Whatuwhiwhi, Karikari Peninsula, Doubtless Bay, Far North, Northland, New Zealand, nz) are the best option, and Andre is a great guy. You might just have to take some pronounciation lessons to work out who to say the name of the town, though!!
The other must dive is the Mikhail Lermontov - a Russian cruise liner that sank in 1986. She's in about 38m of water, and has got to be one of the largest accessible/diveable wrecks in the world. Well worth the trip.