The ACCC has warned energy generators that they will have to pay back money collected for the carbon tax, if it is repealed.

Energy generators have been charging retailers and, therefore, customers, for the cost of the carbon tax, but they do not have to pay up until 2015.

This means that most of the generators have gathered windfall amounts in the millions of dollars because their contracts already include a fee to cover the tax in the future.

But ACCC chairman Rod Sims says that if the tax is repealed by a new senate in July, generators will have collected the money for no reason and will have to work out a way to pay it all back.

In a situation he describes as “messy”, Mr Sims has told Fairfax Media outlets that companies will not be let off.

“They could be breaking the law, so they need to find a way to deal with this,” he said.

Mr Sims says the message to the big power generators is clear.

“If you continue to charge a carbon component past July 1, then we’ll be knocking on their door to say you need to find a way to pass it back because a windfall gain is inappropriate.

“We are asking the industry to work out ways to make sure that money gets refunded.”

But the reimbursal is not that easy to work out. Negotiations between generators, regulators and intermediaries have not seen any great breakthroughs so for.

Each group is keen to preserves its interests, and with the repeal still not confirmed, no one wants to give up the higher rates they are allowed to charge for now.

Talks will continue until the new senate is formed and votes on the future of the carbon tax.

A proposal from the energy industry, on how it will pay back the millions of dollars to retailers and potentially to customers, is expected in coming weeks.