Energy Minister Megan Woods given assurances blackouts aren't expected tonight

Energy Minister Megan Woods says she has been given assurances blackouts aren't expected to occur again on Tuesday night.

Tens of thousands of North Island households were plunged into darkness on Monday night after demand for electricity reached an "all-time high".

Transpower, which owns and operates New Zealand's national grid, said in a statement it did not have enough generation to maintain the significant demand.

Therefore distribution companies were asked to reduce the load by undertaking rolling power outages - which came on one of the coldest nights of the year.

During question time in Parliament, Woods confirmed she had been assured blackouts would not happen again on Tuesday night.

"I have been talking with officials and seeking assurances from across the sector on what is in store for New Zealanders tonight," she said.

"I would like to reassure New Zealanders that I have received those assurances from Transpower. I have had an assurance that 540 megawatts of spare capacity in the system tonight so we are not looking at a repeat tonight. 

"But given how quickly the situation changed yesterday, this is a situation I will continue to get assurances on."

The situation has been heavily criticised for its lack of warning to customers. Transpower only issued a statement to Facebook at 8:56pm - just four minutes before the situation was expected to be resolved.

Just an hour earlier, some distributors urged medically dependent consumers to action their backup plans or go to hospital for assistance.

Woods said market failures were behind the blackouts.

"The situation that unfolded yesterday was unacceptable. If we are going to have a market-orientated system providing security of supply then that market must deliver. The market failed in this respect."

She also questioned why the HVDC cable - the 'Cook Strait cable' connecting the North and South Islands' electricity networks - was running at just 50 percent capacity and why the third Rankine at the Huntly Power Station wasn't fired up. 

She said an investigation is now underway into the incident and she is demanding answers.

"While we cannot undo what happened yesterday, we can thoroughly investigate the cause and make sure it doesn't happen again," she said.

Transpower operations general manager Stephen Jay apologised to those affected on The AM Show on Tuesday morning.

"We will need to look into why there wasn't enough generation available last night. There is normally some underlying reason in terms of why there is insufficiency out there. That is part of our ongoing investigation to look into that."