Hey mate,
I did the north island just over a year ago and was blown away, literally as well as figuratively. I've never been to the south Island but everyone raves how good it is compared to the north. All I can say is it must be out of this world as the north is pretty amazing. Only negative is whilst driving around you always have to stop too because you need to take more photos. Truly a special spot. If you are going for wind and waves then obviously hit Raglan and follow it down to Taranaki? Some really good setups along that stretch.
Best bet if flick Plummet a PM. He gave me some really handy hints and tips before I left last time, a wealth of knowledge.
Raglan, Kite the arvo

Raglan, surf the morning
Taranaki
A must stop!
Plum's wife. ( I had to for Clauncy )