I'd grab a vodafone pre-pay sim card. That's what I use and have been happy with it..

http://search.dse.co.nz/search.php?p...nd&isort=score

or the whole package:

http://search.dse.co.nz/search.php?p...nd&isort=score

If you want to get a used phone: http://www.trademe.co.nz/Mobile-phones/index.htm