One of two generators at the gas fired power station in July experienced an electrical fault, which Origin has informed AEMO that it intends to fix by December 20.

But AEMO's 2019 Electricity State of Opportunities report released this month stated there was a "60 per cent probability that the Mortlake unit outage will extend into the summer months" until March 2020.

The report also predicted a 30 per cent likelihood that AGL's Loy Yang A coal-fired power plant in the Latrobe Valley would remain out of service for the same time frame.

AMEO said if both outages extended over the summer, and no additional supply was secured, extreme weather events could cause between 260,000 and 1.3 million households to be without power for four hours.

An Origin spokeperson told The Standard that the energy company was working to have the damaged unit in Mortlake servicing before December 20.

It is understood the predicted timing in the AEMO report is not based on information Origin provided.

Victorian Energy Minister Lily D'Ambrosio has written to Origin to make her expectations clear that the infrastructure needed to be ready by summer.

Ms D'Ambrosio said the report confirmed risks this summer, and pointed out the market operator was "hamstrung in planning for the future" which she pinned down to "archaic" national rules.

"You only have to have a look at the history, especially in recent years, of the projected return to service of these privately owned and managed units, they continue to overshoot the date of expectation and we are seeing an increasing delay in return to service," she said.

"What confidence can the market operator have when the private owners of these assets say they will return on a certain date.

"The market operator have identified they do not have the rules in place to put in place contingency plans."

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