I took more than I needed, but the least needed item was my wet weather stuff. Only used it while in North Island on SH to Napier from Taupo. I don't think the temperature was ever below 20c on average. It was such a good trip, the roads where hard on the tyres though....time for Metzellers.
If you still haven't left yet don't worry. I thought I was packed pretty light but still took stuff I never used (heated jacket liner, thermal underwear and guidebook were most significant useless items) and didn't need anything I hadn't got with me. Take sunglasses and/or tinted visor if you have them. Don't forget sunscreen and insect repellent (I didn't use the latter but would still strongly recommend although you could buy it there - Note that DEET may not be most effective repellent for sand fleas which are the nasty biters)
enjoy mate.........great roads down there..........