Yes, the weather is a factor over there. February and March, and into April would be the best riding months in NZ. Just wait until I've retired back home and I'll plot out some itineraries for you. If you're going over next year (Jan/Feb 2015) PM me and I can recommend a few routes in the Auckland to Taupo region of the North Island. There are so many fantastic roads, and they're not too far apart.
Get a copy of Mike Hyde's "Twisting Throttle New Zealand" out of the library. After riding around the edge of Australia (Twisting Throttle Australia) and through all the states of the U.S.A (Twisting Throttle America), Mike Hyde rode around NZ and documented his best 50 roads - there are a few roads best suited to adventure bikes, but many, many excellent roads for a sports touring bike.
I have just read in a motorbike magazine that, "If you're visiting only one overseas country, make it New Zealand. If you're going to visit two, make it the North Island and South Island of New Zealand."