If it's unlocked, then I'd go the Gophone naked SIM $1/day (if you make a call) plan. Cheap for making a few calls, isn't outrageously expensive for a (fex, quick) foreign calls, can do Skype, and you can add data in chunks as you need it. SIM card will cost $25 (but you get a $25 iTunes card in exchange, so it costs nothing really), then get $25 credit and channel most of that into data. You'll get 3G, decent coverage, and a known quantity (just search for the IPN downloading site in NZ before you go, and bookmark it).
Just don't tell then it's for an iPhone!