The company had expected to repair a damaged generator before December 20, but it has now extended that date until December 30.

An Australian Energy Market Operator report previously predicted a 60 per cent likelihood that repairs could extend until March 2020.

AMEO said if that was to occur, coupled with repairs occurring at AGL's Loy Yang A coal-fired power plant in the Latrobe Valley, extreme weather events could cause up to 1.3 million households to experience power outages this summer.

But Origin executive general manager energy supply and operations Greg Jarvis said works would be completed "in advance of the traditional peak summer electricity consumption period".

"We've had crews working around the clock to return the damaged Mortlake power station generating unit to service," Mr Jarvis said.

"Unfortunately, inclement weather and delays in securing specialist commissioning technicians have impacted our schedule."

A spokesman confirmed the delay of a superload on south-west roads in October that was carrying a generator had also contributed to the holdup.

The two generating units at the station both have a generating capacity of 283 megawatts, each representing less than three per cent of Victoria's total installed generating capacity.

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