If you happen to be travelling to NZ as a tourist, replacing local sim cards would be the cheapest option to communication and be online.
2 Degrees Tourist SIM is a SIM card offered for tourists coming to NZ and is sold through tourist and Duty Free stores including JR Duty Free. It offers standard rates for use in New Zealand (44 cents per minute to call other NZ mobiles, 22 cents per minute to call NZ landlines & 2degrees mobiles, 0 cents per text, 50 cents per MB of data). You can still purchase Value Packs using this SIM card. If you want a normal 2degrees SIM card, they're available from dairies, supermarkets, petrol stations and a number of retail outlets across NZ and cost $10.
The current $19 Combo packs is probably the best option recommended as it offers 30 min talk, 300 texts and 300MB National 3G data. The coverage area includes major cities and highway roads throughout NZ.
Another option I like about it is the ability to connect to facebook.com for free, so you can update your status, view your News Feed, like or comment on posts, send and reply to messages, or write on your friends' Wall absolutely free.
Be sure to keep up to date with their latest news on http://www.facebook.com/2degreesmobile.