New Zealanders are being told to reduce their power usage on Friday morning or face outages.
Transpower said there is the potential for a shortfall of electricity supply between 7am and 9am because of an "unseasonal cold snap" and low wind generation.
In a statement on Thursday, the state-owned transmission company urged people to be "mindful of their electricity use" to "reduce overall demand and help towards ensuring there is power available for all".
Transpower told people to avoid charging appliances or electric vehicles and using the washing machine, dryer or dishwasher during that time period.
People should also turn off heaters and lights in rooms they aren't using until 9am, it suggested.
"It is forecast to be a very cold morning and people should stay warm by heating rooms they are using and continuing to keep them warm," Transpower said.
"However, they could consider turning down the temperature slightly between 7am-9am (by 1-2 degrees)."
A warning notice has been sent advising the industry of the situation and Transpower is working with electricity lines companies to switch off controllable load like hot water systems on Friday to reduce demand, it said.
Transpower said hot water systems are regularly switched on and off during the day so most people won't notice.
Major industrial electricity consumers have also been asked to reduce use during the same time period.
"If action from lines companies and major industrial consumers, alongside support from New Zealanders to conserve electricity, is not enough to keep the power system in balance, Transpower may need to work with lines companies to switch off supply to some areas around New Zealand," it warned.
"This would be for a short time until the electricity supply shortage is resolved and will prevent the risk of widespread and longer outages."
People who rely on power for medical reasons are asked to "ensure they have a backup plan in place and call 111 in an emergency".
"We encourage people to check websites and Facebook pages from Transpower, their electricity retailer or local lines company for updates."
It comes as the country braces for a frosty start on Friday, with forecasters warning temperatures are expected to plummet overnight. Aucklanders can expect temperatures as low as 1C while others should brace for temperatures well below 0C.
Earlier this week, MetService warned Kiwis to "grab the blankets and hot chocolate" with icy weather set to chill parts of the country.
In a forecast video shared to YouTube on Wednesday, meteorologist John Law warned a polar blast is on its way - bringing the risk of snowfall to higher passes across the South Island.
