The Labour Party claims that power companies are unfairly charging poorer customers higher rates through their prepay plans.
The party has obtained figures from the Parliamentary Library showing the percentage difference between prepay and standard retail rates throughout the country.
The figures show that prepay customers are paying between 12 and 60 percent more than those on standard retail plans - with people in Invercargill and Dunedin paying the most.
Labour's energy spokesperson, David Shearer, says people who have difficulty paying their bills are often put on prepay.
"I think this is absolutely wrong," he says. "These are the people who are generally the poorest in our society. They have no other option and they're effectively being screwed by power companies who are taking more profit out of these people than they should."