Trustpower is apologising to over 100 customers who were told they would have to pay more for electricity.
Community relations manager Graeme Purches says Trustpower has been writing to customers whose consumption is below the typical level for the low use tariff to check that the property is their permanent residence.
Some 191 letters were sent to addresses which used less than 12% of the average household consumption, saying they would be moved to a higher tarrif within 30 days.
Mr Purches says those letters were poorly worded and should have asked them to confirm the address as their permanent residence.
Trustpower has now written to apologise.