Mighty River Power is seeking a price correction, after estimating a massive price spike on Saturday could reduce its earnings by up to $25 million.
The company said it would be lodging a notice of undesirable trading situation with electricity market regulator the Electricity Authority.
That followed a period of about sixhours on Saturday when Mighty River was exposed to buying electricity at wholesale prices more than 200 times higher than normal prevailing prices, Mighty River chief executive Doug Heffernan said.
The undesirable trading situation process was an established mechanism for addressing abnormal market conditions, such as those on Saturday.
During maintenance outages on the national transmission grid in the upper North Island, market prices increased to about $20,000 per megawatt hour (MWh), more than 200 times higher than normal prevailing prices of less than $100MWh, Mr Heffernan said.
That could have a financial impact of up to $25 million on earnings before interest, tax, depreciation, amortisation, and financial instruments, unless the prices were corrected.
Mighty River would be pursuing a correction of the prices as provided for under the existing electricity market rules.
If successful that would eliminate any material impact on the company's earnings for the current period.
Transpower chief executive Patrick Strange said the rise in prices happened during routine Transpower maintenance.
Plenty of generation was available on both sides of the constraint during the work.
"The power system was perfectly secure.
"What you've seen is a pricing matter, which we can't comment on," Dr Strange said.
"We as system operator and grid owner, would not continue with a constraint if the power system security was at risk, ie there wasn't enough generation."
Transpower gave plenty of notice of its maintenance work and did it at times when it had the least impact.
