Unions at the giant Yallourn mine and power station in Victoria’s east are demanding a closure agreement with the plant’s operators in case of a shutdown, arguing coal-fired power stations are on "life support".

The move raises the prospect of another seismic shock to Victoria's energy supply, which is still absorbing the closure in 2017 of the Hazelwood generators, plunging the electricity market into chaos and throwing hundreds of people out of work.

Yallourn power station is expected to close. Credit:Joe Armao

Unions representing the 500 permanent workers at the Latrobe Valley plant say they have learnt from the "ambush" of the Hazelwood closure and want a deal with Yallourn's owners that looks after their employees when the station closes.

The Yallourn workers' demand for certainty echoes the the Andrews government's lack of confidence in the coal sector.