Hey Rich,
You'll have an awesome time. The diving in NZ is fantastic, with lots of variety to suit all tastes. The Poor Knights are a definite must see, other places to consider are White Island/Volkner Rocks (out of Whakatane), the wreck of the Mikhail Lermontov in the Marlborough Sounds, Fiordland, the Rainbow Warrior..... the list goes on, where ever you head there will be some great diving nearby.
Tutukaka is very friendly, especially in summer. The marina has a fairly transient population of boaties, and the locals are very used to opening arms to all. Which ever charter boat you use, the crew will tend to invite you for a drink after the day's diving and you'll have a blast!
Diving wise, we tend to get a couple of algal blooms in early summer - the timing is difficult to predict, but in my experience there tends to be one in november and one in december, though earlier rather than later. By the end of december it tends to have cleared up and good vis is much more likely. Saying that, even in the worst of pea soup blooms the vis tends to not frop below 8-10m, and a lot of the really fun stuff at the Poor Knights is the encrusting life - so you can have an awesome dive just by getting nice and close to the walls.
If you're looking at renting a holiday house (introduction to localspeak - known as a "bach", pronounced batch) then you will probably want to book early for that time of year. A lot of people who own bachs will will stay at home for Christmas then head away after Boxing Day with the family. Some useful websites for finding places to rent are:
Bookabach
Holiday Houses
You've got a lot of options when it comes to diving - the main operators I use for day trips are Dive! Tutukaka (
Dive! Tutukaka) and Yukon Dive (
Yukon Charters), though there others and who ever you speak to will have an opinion. If you're not bringing your own dive gear, Dive! Tutukaka are probably the best set up in terms of rental gear etc.
I'd also think about doing a couple of days as a liveaboard trip, you get more diving in that way. It's also a good option for not having to struggle to find a bach for New Year! There are two main liveaboard options, Pacific Hideaway (
Pacific Hideaway NZ) and Ocean Blue (
OceanBlue Adventures). I expect that Pacific Hideaway is already booked for New Year by one of the local dive shops, but you might be able to get on to the trip. Phone Mark and Denise (they run PH) to confirm, but I expect Global Dive in Auckland have the trip booked.
Kevin and Janine who run Ocean Blue are also great to dive with, they are less likely to be booked and have a smaller boat which only sleeps 6-7. If you are interested in a liveaboard with them, let me know - I can try and help arrange something and fill up the extra spaces.
Water temperature at that time of year will probably be around 19 degC.... it's still warming up. Most people will be diving 7mm wetsuits, because I am soft I'll be in my drysuit still. It will still seem pretty warm if you are coming from the UK, though!!
Most of the people who post here will chip in and give you some advice - diveChiq, Azza, Packhorse, Plumb_bob etc. If that's not enough, you can check out some of the smaller NZ dive forums such as:
www.scubadive.net.nz
sods :: Index
or feel free to drop me a PM with any specific queries.