Ok it cost me some cents but hopefully this information saves you a few in return.
The default APN for Vodafone NZ in the 3.0 software is:
Contrary to what is logical, if you change this to ANYTHING (ie. make a fake/dummy APN), then you WILL connect and be charged casual data if you are on Prepay and not on an iPhone plan. I am unsure what happens if you are on 100Mb/month because I don't have this add-on.
Paradoxically, when I LEFT the APN as is, then I was unable to connect and there was a pop-up error message to this effect.
So if you have a 2G iPhone on Prepay, then just leave the default APN to BLOCK data, change it to something else (eg. remove the "z" from "nz") and you can then use data.
This applies to the 2G iPhone on Vodafone Prepay without any data plans (casual data $1/day on days used).
I am on a legacy Prepay plan (Motormouth).
$1 later, and I discovered that regardless of dummy APN or www.iphone-vodafone.net.nz, it would STILL allow me data access :/
PhilipRoy: This has always been my experience and there used to be a note on the unlockit website that warned you of that. As I said previously....