Two big electricity companies are complaining about a sudden rise in electricity prices while power lines were shut down for maintenance over the weekend.
Supplies to the upper North Island were restricted for about six hours, and Meridian Energy says its fellow state-owned rival, Genesis, took advantage by quadrupling the price it normally charges for electricity from its Huntly power station.
Meridian says it could lose a lot of money as a result.
Another state-owned generator, Mighty River Power, has put a figure of $25 million on its potential loss.
Mighty River chief executive Doug Heffernan says it's not unusual for prices to go up during an outage but "we've never seen prices of this order".
Both companies have filed complaints seeking redress from the industry regulator, the Electricity Authority.
Genesis Energy is refusing to comment, saying the authority is investigating.