Two more power providers have revealed they are putting up their prices, with some customers facing an 8 per cent increase in their electricity bills.

Mercury says prices will rise for about 200,000 Auckland customers by an average of 3.5 per cent on April 1 as part of an annual review process.

And about 246,000 Contact Energy customers nationwide will be paying, on average, between 6 and 8 per cent more.

The regions facing a 7 per cent increase from March 1 are Auckland, North Shore, Northland, Counties Manukau, Thames, Kapiti, Manawatu, Tasman and Invercargill.

Wellington, the Eastern Region and Hawkes Bay customers face rises of 8 per cent and Christchurch customers will have a 6 per cent increase.

The companies say they are passing on the costs from what they are spending on power generation facilities so that New Zealand, which has growing demands for electricity, can continue to be supplied with enough in coming years.

However, an upset Mercury customer, who asked not to be named, described the price increases as a "swindle".

The elderly man said that while the price rise might not seem like much to some, it made a big difference to those on a pension or fixed income. He said he was going to start looking for another provider.

Mercury general manager James Munro said the rise would add less than $5 on average to residential customers' monthly bills.

He said the increase reflected the need for continuing significant investment across the industry to ensure New Zealand consumers continued to have access to enough power when it was needed.

Electricity Authority statistics showed that demand for energy was forecast to rise by about 75 per cent over the next 40 years as the population and economy grew, he said.

"As a country, we have to continue to invest to make sure our homes and businesses have the energy they need, so the New Zealand economy can grow without concern about a shortage of energy. That flows through to all of us in our electricity bills," Mr Munro said.

The company will next review its standard residential electricity prices in April 2012.

A Contact Energy spokeswoman said the company had spent almost $600 million on the Te Huka geothermal station near Taupo, the Stratford Peaker power station, which is nearing completion, and the Ahuroa gas storage facility, which is also in Taranaki.

The current retail cost of electricity did not reflect the true cost of generating and supplying it, she said.

Genesis customers have been told their power will rise by an average 3 per cent.