The federal government is keeping up pressure on AGL Energy to sell its ageing Liddell power station, despite confirmation that the only company thinking of buying the plant is no longer interested.

Josh Frydenberg, the environment and energy minister, said the "only option ... currently on the table to ensure there is no supply shortfall in 2022 is the continuation of Liddell as an operating plant".

"If AGL believe Liddell is a worthless asset then they should test the market and put it up for sale and allow interested parties to undertake due diligence on the plant."

Mr Frydenberg was responding to comments from Delta Electricity on Monday that but all but ruled out any purchase of Liddell. They came less than two weeks after Delta said it would be prepared to conduct a "thorough due diligence" on the 46-year-old station.