Transpower is asking people in Christchurch, Rangiora, and Kaiapoi to conserve power immediately amid a cold snap to prevent power cuts.
Earlier this week MetService issued a heavy snow watch for Queenstown Lakes and Central Otago, and warned southern and eastern parts of the South Island would see highs around a wintry 12C on Wednesday.
But with temperatures already plummeting to single-digits on Tuesday evening, Transpower said electricity demand was “soaring”.
Add to that a planned outage at a substation in Islington, and there was a risk of unplanned cuts.
Transpower said people should put off using washing machines, dryers and dishwashers, delay charging electronic devices - including cars - and turn off heaters and lights in rooms they were not using.
“Transpower is advising people to stay warm by continuing to heat the rooms they are using but asking them to consider turning down the temperature a degree or two,” the national grid operator said in a statement.
“Transpower is working with electricity lines companies Mainpower and Orion to switch off controllable load such as hot water systems to reduce demand on the grid.
“If this is not enough to balance the power system, along with consumers conserving power, then Transpower will need to ask lines companies to disconnect a small number of customers for a short time until peak demand is over, which is expected to be around 7pm.”
Transpower said this would be a “last resort” used to prevent “cascade grid failure, which would result in extended outages for a much longer period”.
At the time of the warning, Christchurch was down to 7C and Dunedin, 8C - the latter now not expected to reach double-digits on Wednesday.