OK, well there are afternoon harbour races (called rum races) on Wednesdays and on Fridays and I'm fairly confident that you could find a spot on a boat for at least one or two races. Rum races happen throughout the year.
On weekends there is generally harbour racing of some sort going on and crew can be a problem for some boats, but weekend racing tends to be more serious than the rum racing.
Its our summer at the moment so there are longer races and passage races going on, once again you may be able to pick up a crew spot.
Failing that, there are various boat owners who zip off for a weekend on the harbour or the Hauraki Gulf and I'm sure some of them may wish to have new people along. You could try posting on the Island Cruising Association website (a NZ association of cruisers) or on Crew.org.nz (a NZ sailing forum much like this one, though slightly more irreverant). You can try yacht clubs like Ponsonby Cruising Club as you've indicated. Try emailing firstname.lastname@example.org
We try to slip out on our boat as often as we can, but its often at short notice when we can make enough time amongst all the other stuff we have to do.
Let me know if I can help you further.