I would echo Rambaldi's recommendations above for operators.
Northland Dive in the Bay of Islands is sort of like doing a land-based liveaboard.
Ocean Blue (Mazurka) is awesome for a liveaboard at the Poor Knights Islands - for me personally the day of diving is never done and I am in love with night diving at the Poor Knights (a liveaboard gives you this opportunity). Yukon Charters and Dive Tutukaka are also fab for Poor Knights diving.
There are definitely some wonderful attractions up north:
- Ninety Mile Beach
- Cape Reinga and lighthouse
- Tane Mahuta and kauri forests (kiwi spotting at night time in Trounson)
- plenty of lovely day walks: undulating coast, sand dunes, forest
- kayaking trips
But you can't expect to see a country from top to bottom in one hit so you may need to decide between diving and some of these other things. Travel time in NZ is slow so fitting in much after a day of diving will be difficult.
Water temperature in late January will be warmer than Rambaldi suggests. 14-15 Celsius is our winter water temperature on the upper North Island. The water temperature starts edging up from now onwards into summer, but lags behind the season due to the time it takes to heat up the ocean. By late January we should be headed towards our summer high water temperature. For the Northland east coast this will be around 20 Celsius, the Poor Knights Islands is usually about one degree warmer than the coast and will generally hit a high of 21-22.
Personally it is a drysuit water temperature all year round for me if you want to be comfortable on long dives and consecutive dives in one day. Try Global Dive in Auckland for drysuit hire. Most kiwis dive wetsuits because they're too tight to spend their $ on a drysuit, not because the water temperature doesn't warrant it.
If you have any more questions, please fire away. I could dive in a ditch and enjoy it but in my opinion New Zealand diving is awesome and you are going to love it!