The corporate regulator is requiring a second electricity lines company to reduce its future earnings because it over-charged customers.
The Commerce Commission said Horizon Energy over-charged its customers by nearly $645,686 in the year ended March 2012.
To compensate Horizon must reduce its earnings by at least $727,934 in the year ending March 2016.
The commission said this took into account the degree to which Horizon benefited, as well as the fact that the company made attempts to remediate some of the breach, which hadn't been intentional.
Commission deputy chair Sue Begg said this was not the start of a trend, but it does send a signal to all electricity lines companies that there will be consequences if they over-charge.