Mobile companies

There are a few major companies, Spark, Vodafone, 2Degrees and Skinny. Prepaid is where it's at.

Spark
Has near-complete coverage of New Zealand, if you're going to be outside of towns or cities, then Spark is your best bet for retaining signal.

2Degrees
Widely reputed to be one of the cheapest companies to be with. Outside areas of 2Degrees coverage, coverage roams onto the Vodafone network at no extra cost.

Skinny
Skinny is a division of Spark, they offer cheap prepaid services on monthly and weekly basis.