I would not recommend buying a phone in NZ. Maybe there's some specials or deals I'm not aware of, but I found them to be extremely expensive, and that was before the dollar dropped a lot more against the NZ$.

I have bought cheap (really very cheap) GSM phones w/SIM card in the UK for $15 and the US for $20, but the same thing (literally the identical phone) cost me NZ$100.

If you rent or buy a phone here before you leave, make sure it's at least tri-band GSM; regular US GSM phones don't work there either.