Genesis Energy customers are about to be hit with a rise in their power bill.

The company is sending letters to about 62,000 customers advising them of the change, which comes on the back of price rises in October, when power companies pushed prices up because of a 2.5 per cent rise in GST.

The company says that the average increase for customers is 3 per cent.

But Karyn Scherer, who lives in Titirangi, said the increase on her power bill was going to be much more than that.

Her bill, which is on average $180 per month, is set to rise by about $35 - a 20 per cent increase.

Ms Scherer's bill is charged on a combination of the company's variable night-rate plan and variable day-rate plan which allows her to use appliances such as her dishwasher and clothes dryer at night, when power is cheaper.

But the variable night-rate plan has increased the most out of the company's seven household pricing plans, going from 10.37 cents per kWh to 18.56c - a 79 per cent rise.

Genesis spokesman Richard Gordon said the night-rate rises affected about 850 customers.

"During the process of this pricing review we discovered that these 850 customers were being substantially undercharged for their night rate and were on an incorrect tariff ... this has now been corrected but has resulted in a significant increase for this portion of their electricity bill.

"However, we do aim to provide fair and reasonable pricing to our customers and this new night rate is still cheaper than our competitors. Overall, prices have increased on average by 3 per cent - the last price change was in May 2010.

"This is due to increasing costs to run our business plus some increase in the distribution and transmission portion of the tariff."

Molly Melhuish from the Domestic Energy Users Network said she was concerned about Genesis' 23 per cent increase in its variable-controlled rate.

The variable-controlled rate allows power companies to save money by, for example, turning off customers' hot water cylinders at set times.

Mrs Melhuish believes increases in the price of plans allowing for cheaper night rates was a sign that power companies were trying to steer customers to one pricing plan, which was not encouraging sustainable power use.

"I feel there's a real move in the industry to get consumers to be passive and just accept what they're given. This is a move to simplify rates so there's no advantage to a night rate ... it is a serious insult to sustainability."