Looking at:

http://www.gsmworld.com/roaming/gsminfo/cou_nz.shtml

NZ has vodafone with GSM + 3G 2100, and Telecom NZ at 3G 2100/850. So both will work when in urban areas (2100), but vodafone will work better in the woods (or should I say in the middle of sheep:P) .

A minor detail: wifi channels in US are more restricted than in elsewhere. You won't be able to connect to Access Points on channels 12-13.