Use triband phone in NZ?

GSM900 and GSM1800 exist in Australia and New Zealand, so on the radio side, a triband GSM phone will work.
Just check with your operator the possibility of roaming with your SIM card. You should also check the prices, especially the price differences between Australian & New Zealand operators as it can be quite expensive.
 
I'm with O2 and my tri band works fine in OZ.....not sure about NZ...a hefty bill arrives home before you do !