Which provider? I noticed (too late) that the airport SIMs have special overpriced plans. I'm used to paying ~$10/GB for data in Australia, it seems to be about double that in NZ without going on a plan.
I bought a 2 degrees one, but I'm pretty sure they had Skinny and Spark as well at the local New World. Easy to setup with just a credit card, or could've used one of the voucher cards which I'm pretty sure can be bought with cash.
Both here and in the US, I've found that airports usually are not good places to get SIMs - there seems to be some agreement whereby the expensive ones aimed at tourists are all that's available. Supermarkets or department stores tend to be my go-to.
