The company I work for - Truphone - is pretty good if you are looking for a cheap way to call back to the UK.

It is an application that you can download to a lot of different phones, including the iPhone, a lot of different Nokias, for free.

Once you have a Wi-Fi connection, you can make calls over the internet using this applications. Calls are just a few pence a minute, and free if the person you are calling also has Truphone and a Wi-Fi connection.

Good luck with the move to NZ - it is a fantastic country (and as an Aussie, that was a hard thing to say!)