Whilst Auckland is well placed for most things, one thing it hasn't got is fabulous diving right on it's doorstep..... so it depends on what you mean by "close", and whether you want to do shore divesor will be willing to pay for a charter!
Goat Island is probably about the only half decent shore diving near Auckland, it's about an hour drive north. Goat Island Dive also offer boat trips to the inner and outer Hauraki Gulf. There are some good dive sites here, but in my experience often passed by in the need to cater for the hunter gatherers.
A couple of members of SB have their own boats, which makes getting out from Auckland a bit easier.
The best diving is a 3-5hr drive out of Auckland. My personal best dive site in the world is Volkner Rocks, a 5 hour drive to Whakatane and a couple of hours out on the boat. Baker Marine Charters offer liveaboard trips to dive Volkners and the nearby live volcano, White Island.
Slightly closer, and more famous, are the Poor Knights islands about a 3 hour drive north. To come to NZ and not dive the PKs would be a shame.
Expect cool water, temperatures in August will be somewhere between 14 degC (coastal) to 16 degC (if you get offshore into the warmer water).
If you've got any specific questions, feel free to fire away.
