Liddell power station has lost a third of its capacity following another turbine failure.
Turbine four was taken offline on Monday due to a leaking hydraulic tube.
An AGL spokesman confirmed the breakdown. It is likely to take a week to repair the turbine.
The remaining three 500 megawatt turbines only returned to full capacity on July 20 following a generator malfunction at the start of the winter peak period.
The average wholesale price for electricity in the National Electricity Market, reached $264 per megawatt hour in the June quarter, double the previous record, three times the price of the same period last year, and more than five times the average of 2020.
AEMO has called for an accelerated shift to wind and solar, supported by batteries, to help drive down skyrocketing power bills.
The failure of several coal-fired power generators, including Bayswater, in early June, led to the suspension of electricity spot market in NSW, Queensland, South Australia, Tasmania and Victoria
Liddell represented a new era of electricity generation in Australia when it was switched on in the winter of 1971. It remained the state's largest generator until the construction of Bayswater and Eraring power stations in the mid-1980s.
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