I'm not from new Zealand but i've been there two years ago in Dec. and Jan.
2 years ago there was no real kitespot in Aukland. All the people went up to OREWA which is about 30 km further north on the east coast... That seemed to be a nice place with lots of space, no currents, etc... There 's also a shop located : http://www.anabatic.co.nz
They have their adress and phone number online... I would give them a call thei can definatly help u further. and since there is still quit less kitesurfers in New Zealand they all help together and everybody who wants to visit them as well!
Also found a nice spot guide for u here: http://www.ikiteboarding.com/kiteboardi ... d/165.html
There is other great spots in New Zealand like bay of islands, 90 mile beach, wellington, RAGLAN, CHRISTCHURCH and ABEL TASMAN NATIONALPARK,... but they r to far away from Auckland anyway...
I think u 'll need to hire a car (possible everywhere in NZ, just search the web to find a cheap one!) Enjoy Auckland, it's lovely!
Have a great time