With all the recent plans announced from 2 Degrees, Spark and Skinny New Zealander's are finally getting some value for money with the now available smartphone plans. They could all offer more data as the need for SMS and Voice services are needed less these days.
I myself have been on a pre-paid combo pack with 2 Degrees that fits my needs (developer working from home). Even with a small monthly data allowance I've racked up 5 GB of data for when I need it, plus 100's of minutes.