As most of you will know, using a mobile phone while overseas can get bloody expensive, especially if you are using data for things like social media or email.
Might I suggest that if possible, you take a second mobile phone with you and buy a pre-paid SIM card when you land in NZ. They are cheap through 2 Degrees or Spark (the old NZ Telecom) and you can buy them even before your pass through customs at the airport. From memory, I think I paid $5 for mine, and you can add as little as $10 credit on it. Just make sure you get a SIM card that doesn't auto-renew.