I have only dived a wetsuit in NZ (I'm another kiwi) and would say go wet - I normally wore a 5mm farmer john and was toasty in 17 deg water (August).
If you want an update on current water/weather at the Poor Knights, go to
www.scubadive.net.nz :: Index - there is always a local from the Tutukaka area there who will be able to help you out.
Cheers,
MBBP