But Contact says the politician's estimate on the amount of power the average prepay customer used was far greater than the reality and his data was skewed.
Mr Shearer calculated a year of electricity for a Contact customer in Dunedin on a standard contract was $2407.
For a Contact pre-pay customer in Dunedin with the same electricity usage it was $3066 a year - 27% more.
In Invercargill, a standard Contact customer paid $2676 a year, pre-pay customer paid $3596 - 34% more.
Prepay generally catered for people who struggled to pay a monthly bill and frequently had their power cut off, Mr Shearer said.
''People in financial trouble are often pressured to go on pre-pay schemes by power companies.''
Pre-pay schemes were beneficial for power companies because payment was made in advance, eliminating financial risk from power companies and providing revenue before a service was provided.
Power companies should charge pre-pay customers less, not more than its other customers, Mr Shearer said.
''The fact Contact continues to hit those least able to pay their bills is opportunistic and unfair ... Meanwhile, this month, Contact Energy will be paying a special dividend of $367 million to its shareholders. Some of those profits were made from consumers who are struggling to afford to heat their homes.''
Mr Shearer said the figures were calculated on the power usage of a family of four, using electricity for the hot water and stove and one person being home all day.
A Contact Energy spokesman said the amount customers paid for electricity depended on a range of factors, including the region in which they lived, the number of people in their household, the level of insulation in their home, how much electricity they used, the plan they were on, and the chosen payment method.
''Mr Shearer has used a high level of electricity consumption in his prepay calculations, far greater than the amount used by many of our prepay customers, which does influence the outcomes.''
Contact pricing varied by region and, on average, its prepay rates were about 6% more expensive than the price for Contact customers getting the 10% prompt payment discount.
The dividend Contact Energy would pay its shareholders was recognition of shareholders supporting a more than $2 billion investment programme including the development of additional generation capacity of Te Mihi and Te Huka geothermal power stations, near Taupo, and fast start gas power station and gas storage facility in Taranaki.
