About a year ago I switched to 150GB naked cable (after a price drop).
First month I was changed $155 (the old price) rather than $95 (the current price at the time).
Quick call to Vodafone and it was fixed.
You can read about it here www.geekzone.co.nz/forums.asp?forumid=40&topicid=119464

This months bill is back up to $155.
Rang Vodafone and was told the $95 was for the first 12 months only. It was not, it was the normal price a year ago. How do I get this fixed.

The following topic seems to be about the same issue, I was slower than others in switching:
http://www.geekzone.co.nz/forums.asp?forumid=40&topicid=143437