[jvoets]

For scuba go to the Poor Knights
That's a real top spt, especially if you've just started out diving.
The Dive! Tutukaka operator is one of the best diving companies I've ever been with.
The wrecks (Tui and HMS Maori) are very nice as well. The maori is more suitable for beginners.

I've been snorkelling at Goat Island, visibility was very disappointing (2 meters top) last january, but it allows for easy access from the shore. I think depth increases as you go around the small island.

The wreck of the Rainbow warrior is amazing. Well worth a visit, but you'll need a 2 hour boat ride to get there.

If you want to stick to freediving, go with a "Swim with the dolphins" trip in the Bay of Islands. They'll provide you with a lot of gear, but NO FINS or weights! So make sure you bring your own. It's amazing to snorkel & freedive with the bottlenoses.

Water temp: north island some 16 - 20 degrees celsius
South island: prepare for a whopping 12 degrees