Thousands of properties are without power in north central Victoria where temperatures are expected to exceed 40C.
More than 2700 AusNet customers have been hit with a power outage near Nagambie on Thursday morning.
The underground cable fault might not be fixed until 5pm.
The outage comes as Victorians are being warned to prepare for hot conditions and an "oppressive" night as the mercury begins its climb into the 40s across much of the state.
The city is forecast to reach 36C on Thursday and the maximum might not be reached until 6pm, depending on how strong a sea breeze forms during the day, according to the Bureau of Meteorology.
Maximums in the low 40s are forecast for other parts of the state.
The Australian Energy Market Operator is also trying to allay fears the state might suffer a power shortage over the next few days.
Chief executive Audrey Zibelman said there were enough reserves to see through the heat.
"We first go to the generators that are in the market, then we go into our reserves and we carry enough reserves to lose two units and then, only then, we would ask for people to reduce their usage, but that's the last point and we're not even close to that yet," she told ABC Radio.
Melbourne will swelter through its hottest night in a year with the temperature dropping to just 29C, before hitting a top of 43C on Friday.
A cool change is expected late on Friday, but it will bring gusts of up to 80km/h, thunderstorms and increased fire risk.
Fire crews are on stand-by while extra paramedics and ambulances will be on call to assist those struggling in the heat.
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