Some Northland residents are overpaying for electricity costs by more than $400 a year, new data suggests.

The Ministry of Business, Innovation and Employment's quarterly survey of domestic power prices shows up to 187,830 households nationwide were with their respective region's most expensive retailer on November 15.

Together those households paid a combined average of 16 per cent more than households with the cheapest retailer.

In Northland TrustPower customers with the Northpower network paid $417.74 more in the year to November than those with the network's cheapest retailer, Nova Energy.


Mercury Energy customers with Top Energy were the next worse off, paying $302.35 more than customers with the network's cheapest retailer, Contact Energy.

The average Northland power bill jumped $99 in the last year - a 4.3 per cent increase - well above the national average.

The study, which relied on Electricity Authority data, was based on an average household using 8000kWh per annum - close to the nationwide average residential consumption. The results were compiled by online power retailer Powershop.

However, TrustPower community relations manager Graeme Purches said the figures were "a load of rubbish".

The way the data was calculated wasn't designed to compare regions, but provided an overall picture. "Trying to do a calculation based on an average household consumption ... provides a wildly distorted picture." The calculations also didn't account for special deals and discounts many companies offered, he said. "The prices quoted, there'd be very few, if any, customers on them. There are only 5 per cent of our customers on standard pricing." TrustPower prices were "middle of the road". About 70 per cent of its customers were locked into fixed contracts, he said.

Powershop's Ari Sargent said customers didn't switch to cheaper suppliers for several reasons, including being locked in to fixed contracts.

"Some customers sticking with the most expensive retailer are overpaying by more than $650 a year. That's enough to pay for the Christmas spread and most of your presents."

Wairoa showed the biggest gap between the cheapest and most expensive retailer at $655.61 a year, followed by Christchurch at $623.65 and $597.17 in Buller.

Waitaki had the highest percentage of customers with the most expensive retailer at 73 per cent, followed by 70 per cent in Tauranga and 69 per cent on the West Coast. Large retailers were the most expensive in 37 of the 45 regions, where there was less competition from smaller players. APNZ