Areas across the North Island have been plunged into darkness on Monday night after demand for electricity reached an all-time high.
Transpower, which owns and operates New Zealand's national grid, said in a statement posted to social media earlier Monday evening it did not have enough generation to maintain the demand.
"Insufficient generation has been made available to meet demand and manage a secure system.
"As a result, Transpower in our capacity as managers of the power system (the system operator) has asked the distribution companies to reduce 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."
The situation was expected to be resolved by 9pm when peak demand passes.
WEL Networks, which operates in the Waikato area, said the outages will not impact individual customers for more than four hours.
"As a precautionary measure, all medically dependent customers are advised to action their back-up plans or go to Waikato Hospital if required.
"We thank you in advance for your patience and understanding, and we will keep you updated."
Unison, which operates the electricity network that serves the Hawke's Bay, Taupo and Rotorua regions, shared the following message:
"Unison is currently 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 Hawkes Bay, Taupo 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."