A widespread power outage which affected thousands of people around the country overnight was slowly being restored from late last night.

But exactly how it happened has left members of the public puzzled.

Electricity authorities announced that demand for power had reached "an all-time high" and they did not have enough generation in the system to maintain that demand.

Up to 20,000 households were plunged into darkness and, in some cases, cold after high winds brought down power lines and cold weather saw power use surge beyond generation capacity.

Transpower NZ issued a note on its Facebook page telling customers that as the managers of the power system (the system operator), it had asked distribution companies to reduce the load across the country.

"Different companies will do this in different ways. Some manage via load control on hot water, some manage via customer disconnections."

Those households affected started losing power about 6pm.

Transpower NZ said later on Facebook that the situation was expected to be resolved about 9pm, "once peak demand for electricity has passed".

This morning, Transpower told Newstalk ZB that no one was available to discuss the issue until after 8am.

A Trustpower representative told the Herald the outages affected parts of Wellington, Kāpiti Coast, Palmerston North, Taupō, New Plymouth, Taranaki, Hamilton, Napier, Hastings, Auckland and Whangārei.

Some people took to social media last night to vent their frustrations about the power cut - with many asking how this could happen in this day and age.

In response to a Facebook comment from a member of the public who put to them that this felt "very last minute", Transpower NZ wrote back: "We issued a warning notice early this afternoon to electricity market participants.

"This will have been the earliest we would have known about it too. Remember this is an electricity market that has to respond to demand and generation changes 24/7.

Outages shown on the Wel Networks' outage map at 8.30pm yesterday. By 9pm, the outages had been reduced to 49 properties without power. (Image / Wel Networks)

"Of course we could not forecast today's usage or generation capacity months ago."

Energy analyst Molly Melhuish this morning warned that surges in electricity demand were only going to become more common.

Melhuish told Newstalk ZB that cold snaps like the one that caused the surge in demand are becoming more common.

Although many homes had their electricity restored late last night, authorities said staff would still be working to return power to some homes until after midnight in some areas.

A voice message on the Trustpower 0800 number said last night 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.

Thousands of households were without power last night, after demand on the electricity system reached an all-time high. (Photo / Mark Mitchell)

"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 said 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 -2C."

Northpower, which manages electricity in Whangārei and Kaipara, said there were no outages on its network.