The Electricity Authority has launched an investigation into a price spike that publicly owned Mighty River Power warned could wipe up to $25 million off its gross earnings.
Mighty River and Meridian want a correction to wholesale prices, which soared to $20,000 a megawatt hour after a maintenance shutdown on the national transmission grid in the upper North Island for six hours on Saturday.
Mighty River says that's 200 times higher than usual.
Both energy firms lodged a notice of Undesirable Trading Situation with the Electricity Authority, a procedure initiated to deal with abnormal market conditions.
Mighty River chief executive Doug Heffernan says that, if successful, the correction would eliminate any material impact on the company's earnings.