Power failures have been affecting large swathes of the North Island overnight after electricity demand hit a record high.
Up to 20,000 households were understood to be without power after high winds brought down lines and cold weather saw power use surge beyond generation capacity.
Power companies have since been told they can begin restoring power to their customers.
A Trustpower representative told the Herald the outages were affecting parts of Wellington, Kapiti Coast, Palmerston North, Taupō, New Plymouth, Taranaki, Hamilton, Napier, Hastings, Auckland and Whangārei.
Many areas lost power about 6pm.
Staff were working to restore power but electricity may not be restored to some areas until after midnight.
A voice message on the Trustpower 0800 number says it "currently has no restoration times" and the company was experiencing high call volumes.
Wel Networks, which supplies power to the Hamilton area, said there had been insufficient generation to meet New Zealand's electricity load demands.
Distribution companies had been told to immediately reduce the amount of load on their networks, it said in a Facebook update.
"This was a New Zealand-wide emergency."
The company earlier warned customers of rolling outages across the region.
"It is unclear how long these outages will be required for, however, it is expected that this will not impact individual customers for more than a four-hour period.
"As a precautionary measure, all medically dependent customers are advised to action their back-up plans or go to Waikato Hospital if required."
The company had since been instructed to restore power to affected areas, it said.
"We do apologise for any inconvenience caused and we thank you in advance for your patience and understanding."
North Island electricity company Powerco said it was also responding to Transpower's request for electricity lines companies nationwide to reduce load on the national grid.
Controlled hot-water systems across its network had been switched off to reduce network load and would be progressively switched back on overnight.
Vector said it used hot water control and battery installations to reduce the load on its network, as instructed by Transpower, and no customers were affected by outages.
A person emailed the Herald to say, "Massive disruption in Hamilton CBD".
Another said power had just come back on in Napier.
Unison, which operates the electricity network that serves the Hawke's Bay, Taupō and Rotorua regions, said it was responding to a Transpower request for electricity lines companies nationwide to reduce load on the national grid.
"This means that we are having to conduct a series of rolling power outages across our networks in Hawke's Bay, Taupō and Rotorua.
"We fully understand that these outages are frustrating and inconvenient, especially given the current cold snap hitting our regions. We will do all we can to minimise the duration of these outages and apologise for any inconvenience."
Transpower says nationwide demand for electricity is at an all-time high.
"Insufficient generation has been made available to meet demand and manage a secure system.
"As a result, Transpower has asked the distribution companies to reduce load. Different companies will do this in different ways. Some manage via load control on hot water, some manage via customer disconnections.
"The situation is expected to resolve at 9pm tonight once peak demand for electricity has passed."
A mother in Hamilton told the Herald: "Power cut that was apparently planned but NO warning until it was already under way. Not great while trying to keep the newborn warm on a night forecast to hit -2."
Northpower, which manages electricity in Whangārei and Kaipara, said there were no outages on its network.