Half of the Yallourn power station in Victoria's Latrobe Valley is offline, placing extra pressure on the national electricity grid.
Energy Australia, which owns Yallourn, said two of its four generating units were offline because of "unplanned maintenance".
The Australian Energy Market Operator has warned of potential blackouts on the east coast because of a shortage of electricity.
An EnergyAustralia spokesperson said while the outage was "unplanned", overall costs for supplying power had increased.
Yallourn produces about a fifth of Victoria's electricity when operating at full capacity.
The outage at Yallourn comes amid concerns about generators withdrawing capacity from the grid to take advantage of higher wholesale prices.
AGL also earlier revealed that an outage at its nearby Loy Yang A power station would continue until "the second half of September".
Doubts generators are gaming the system
Grattan Institute energy program director Tony Wood doubts operators are withdrawing capacity voluntarily.
Mr Wood said commercially it was in the best interests of both Energy Australia and AGL to be at operating at full capacity.
He said suggestions the operators were "game playing" in order to get the best prices were likely unfounded.
"It's not something that could be undiscoverable. There's either a technical fault or there's not," Mr Wood said.
"The companies would be under very significant penalties if they were gaming the system in a way that they're not supposed to."
