Despite an initial admission of liability, my three phone calls, and more than three weeks of waiting they don't seem to be terribly interested in crediting back the over billed amount so I am now weighing up my next options. Admittedly the dollar amount concerned is only $22 so in that respect it's not a big deal, but I am going to keep pursuing it as a matter of principle.
At this point what I'm interested to know is whether anyone else has experienced this problem. In my case the problem is obvious by logging into the 2degrees web site, viewing the data transaction history, and observing that data charges have occurred during the period of the 13th of October to the 17th of November during which time I was within the Wellington region. If the problem is isolated to my account then the Disputes Tribunal is probably the best place to recover the money, but if the problem is more widespread then a complaint to the Commerce Commission may be a more appropriate course of action.
If anyone has any experiences that they wish to share then it would be much appreciated.