WEL Networks says it was given inaccurate information from national grid operator Transpower on Monday night which resulted in it axing power to nearly 18,000 customers.
WEL lights up homes to more than 160,000 throughout the Waikato and a large portion of its Hamilton customers were plunged into darkness.
About 30,000 homes around the North Island, from Wellington to Whangarei, had power abruptly cut after Transpower said demand had reached "an all-time high" and it didn't have enough generation to maintain demand.
Energy Minister Megan Woods has since launched several investigations including calling on the Ministry of Business, Innovation and Employment to determine who and how it should be communicating with consumers to allow people to prepare, as everyone was caught unawares.
Speaking at an urgent debate about the widespread outage yesterday, Environment Minister David Parker revealed that when power companies were asked to "shed" 1 per cent of their load, some companies converted that from a percentage into a megawatt figure, incorrectly.
That meant lines companies shed more electricity than they needed to, potentially cutting off households that could have stayed connected.
"Some of the lines companies picked that up and they verified the correct number and therefore didn't use the other number than was higher than the figure Transpower wanted," Parker said.
Some lines companies, like Auckland's Vector, picked up on the mistake and sought the correct number, but others did not. These companies shed more customers than they needed to.
Meanwhile, in Hamilton, residents were in the dark, getting cold and angry.
"This doesn't seem right at all," one woman wrote on WEL's Facebook page. "I mean humanely who does this... No warning... Freezing cold night..Breathing machines.. Heaters... Affected.. My mother lives in pensioner units.. I went to check on them all.. Its not good and it is not acceptable!"
WEL's chief executive Garth Dibley apologised to its customers, saying it was dealing with "a rapidly evolving emergency situation and communicated with customers as soon as we accurately understood the extent of the situation".
A Transpower spokesman acknowledged it got the allocation wrong which saw WEL dump more power load than needed.
Dilbey said WEL was advised by Transpower there could be "low generation" between 5.30pm and 8pm, at 6.42am on Monday.
By 1pm, Transpower issued a warning notice and at 4.25pm WEL began reducing its hot water load which would have reduced load capacity by 15MW-25MW or 7 per cent.
However, at 6.47pm, Transpower issued a second grid emergency notice asking all networks to reduce load by 1 per cent.
"Under Government regulation we must adhere to instruction from the system operator in an emergency and that is exactly what we did," Dibley said.
"It should be noted that we were instructed to act with immediate effect and did not have visibility of the deficit in generation that Transpower was trying to manage or what has been requested of others."
WEL said it "repeatedly" questioned Transpower regarding the volume of load it was instructed to drop, "however Transpower was insistent on the volume".
"WEL was asked to reduce load by more than 20 per cent, well above the national average."
At 7.48pm, the company put up a post on its Facebook page advising customers of the outage.
Dibley said WEL started restoring power to its customers from 8.10pm and all were back to normal by 8.32pm.
"It should be noted there is regulation that requires Transpower to undertake best endeavours to reduce load evenly across all its customers," Dibley said.
"Transpower subsequently confirmed they had miscalculated the load on our network which contributed to the excessive load reduction requirement."
Dibley said the situation was "completely out of WEL's control".
"We were dealing with a rapidly evolving emergency situation and communicated with customers as soon as we accurately understood the extent of the situation."
A Transpower spokesman said it acknowledged "that the allocations to shed demand were in error".
"Some networks were asked to shed more than their fair share, including WEL Networks.
"We regret the added inconvenience and frustration to consumers on a cold night. We are investigating further in order to understand what went on and to make improvements.
"We do thank those networks like WEL and their consumers that took a larger share of the allocation overall. The action taken helped stabilise the system therefore avoiding a major failure of the power system."
Dilbey said its response was in "direct accordance with the industry rule which is to avoid total loss of power to our country and we acknowledge and apologise to those customers who experienced an outage".
He said they acted in good faith with Transpower's instructions.
Dibley now asked for people to "please be considerate towards WEL staff, who were doing what they were directed to in order to help avoid a much more severe outcome for the country."