Genesis Energy has defended not firing up its third coal-fired boiler unit during Monday night's power issues, saying Transpower's warning came too late.
Investigations are underway to find out why thousands of households were plunged into darkness without warning on Monday night.
Families were left freezing after the grid operator Transpower ordered power cuts to people's homes.
Kerin Watson was bathing her eight-month-old son when her home in Hastings turned pitch black.
"I was unable to see my son, which scared me a lot. I rushed to him, because you know he's in water," she said.
With no heating, she says her home felt like a fridge and her baby was unfed and freezing.
"I couldn't feed him any dinner, I couldn't fill his bottles up, it was horrible."
Thousands of households across the North Island were surprised with deliberate blackouts on Monday night on one of the coldest nights of the year.
Rolling power outages occurred in Northland, Hamilton, Hawke's Bay, Taupō, Rotorua, Kāpiti and Horowhenua.
"Last night it [demand] went up to 7100 megawatts," Transpower operations general manager Stephen Jay said. "That set a new record in NZ for maximum demand ever".
The state-owned company that operates the country's power grid, Transpower notified the industry it could face problems at 6:42am Monday morning - that escalated to a full warning at 5:30pm.
By 6pm it sent another saying "This is a New Zealand wide emergency" and ordered line companies to 'decrease demand: nationwide'.
"Every player in that industry knew what that peak would be," Jay said.
But Transpower and the retail power companies never told customers.
The Government's investigating how it happened - it says it's already found failures.
The system back-up, the Huntly Power Station, was not in full force and Genesis Energy also didn't fire up its third coal-fired boiler unit at the Station.
However, CEO Marc England says that's because Transpower's warning came too late.
"They can't be turned on in a couple of minutes, they take six to 10 hours to ramp up and it was too late for us to bring it into the market at that point."
Prime Minister Jacinda Ardern said "we cannot hand on heart say right now that all of the generation that could have come online, did come online".
"That is a critical question. There may still be a case that this could have been prevented."
Power has been restored but residents in the Bay of Plenty are being told to "turn off non-essential items" like dryers, washing machines and dishwashers until 9pm Tuesday night.
Customers left freezing and fuming just want executives and officials to sort it out. They don't want their children to be cold again tonight.
