Police say a fire at the Yallourn power station in Gippsland in June last year was caused by an act of sabotage.

The blaze shut down the power station.

Three generators were knocked out at the plant which supplies a quarter of Victoria's energy.

It came in the middle of a bitter industrial dispute between the power station operator Energy Australia and the Construction, Forestry, Mining and Energy Union (CFMEU).

The fire coincided with Energy Australia's decision to lock CFMEU members out of the site indefinitely.

Detective Sergeant Alan Rumble says police are asking anyone with information about the fire to come forward.

"As a result of our enquiries we've been able to determine in fact the fire was a direct cause of a sabotage incident," he said.

"It was an extremely dangerous incident, potentially fatal for the person who conducted the offence.

"We are aware that people out there in the community know, have knowledge in relation to this incident, and we're keen for those people to come forward and contact Crime Stoppers."

The Victorian Energy Minister Nicholas Kotsiras has issued a statement saying he is very concerned to read about the allegations.

"No group or individual should be able to threaten the safety of workers and critical infrastructure which all Victorians rely on," he said.

"These matters are being investigated and if found true, should be dealt with by the full weight of the law."

The power station's operator says it will not comment while the investigation is underway.

The union says it is gravely concerned about the revelations.

In a statement, the CFMEU says it does not condone sabotage and it expects police will act if they have credible evidence..