It's just about do-able to get to the Knights and back for a day trip from Auckland - an early start and a long drive, but I do it about every month or so when I get a day off.
In about 3 weeks, I'll know what my teaching schedule is July through November - if I'm off on the 14th August then I could be convinced for a day out if the weather is ok... hopefully by then I'll have either fixed my drysuit or got a new one! You'd be more than welcome to grab a lift if so.
If you're just doing a day trip whilst passing through, I'd look in to Dive! Tutukaka (
Dive Poor Knights Islands. Diving Poor Knights Islands and shipwrecks HMNZS Tui and Waikato with Dive! Tutukaka New Zealand. / Dive! Tutukaka, Poor Knights Diving) who cater very well for the visiting diver. Some of the PK Divers afficionados will no doubt jump on me for daring to suggest anything different - but quite frankly D!T are more likely to be going out midweek in winter than any of the other charter companies based in Tutukaka. D!T offer a free guiding service, which is invaluable for anyone not used to the topography and life at the Knights. The Knights for me is all about the encrusting life, and it does take a bit of time to be able to pick out the cool stuff - without a guide, you'll probably miss out on so much.
Whilst the Knights are some of the best diving here (where are you from, by the way?) they're not the be all and end all - have a look at Goat Island Dive (
Goat Island Dive, Dive trips, Scuba diving courses, Auckland, New Zealand). They don't have their August trips up yet, but places like Sail Rock, Simpson Rock and the Mokohinau Islands offer diving at least as good as the Knights. The advantage is that they leave from Leigh, which is about an hour from Auckland, not three.