Trustpower has been fined $17,000 for misleading customers about a review of power bills.
The company examined all its customers' power bills between May 2005 and July 2007, then sent out 208,000 letters suggesting how people could save money by changing their price plan.
However, the review excluded a low-user option and this was not explained on Trustpower's website.
The letters were also not sufficiently clear in more than 57,000 cases where low-use customers were eligible for a potentially cheaper option, with a smaller fixed charge component.
The Commerce Commission says Trustpower was, in some cases, buying power from distributors at the low-user rate then charging residential customers the standard price.
The company pleaded guilty to breaching the Fair Trading Act.