Households churned through $4 million of extra power over the weekend as the polar blast sent temperatures plummeting across the country.
Consumption around the country surged by 15 gigawatt-hours (GWh) on Saturday and Sunday compared to the previous weekend, according to Powershop.
That was the biggest week-on-week increase of the year.
With an average variable retail energy cost of $0.2638 per kilowatt hour, the extra power used across the country would have cost $4,057,244.
Powershop's Mark Soper said demand jumped by more than 17 per cent in some parts of the country.
"The lower North Island in particular saw a big increase in power use. But these figures show households in all parts of the country were using more power to stay warm and dry."
Consumption increased by 4.99 GWh in the lower North Island, costing an estimated $1,316,362.
Mr Soper said people should plan for higher power costs. Winter could bring a few more surprises yet, so households should check they were getting the best deal in their region.
"Power costs are a challenge for many of us over winter, but when it's this cold households should not have to feel pressured to go without adequate heating," said Mr Soper.
New Zealand households overpaid for power by more than $300 million in 2015 by not switching to the cheapest electricity deal available, according to the Electricity Authority.
Bay of Plenty households stood to save the most at an average of $331 per household.
The lowest average savings were in Hawke's Bay, at $135 per household.
Tauranga Budget Advisory Service manager Diane Bruin said queries around electricity bills were one of the main things the service dealt with at this time of year when bills tended to be higher.
Advisors always told clients to check out the power switching website, www.whatsmynumber.org.nz, but to be aware that their cheapest option today might not necessarily be their cheapest option in six months time.
People should check back with the website at least annually, Ms Bruin said.