Australia remained in the grip of a record-breaking heat wave that continued claiming lives and maintaining its oppressive grip on southern parts of the country on Sunday.

Adelaide is expected to match its longest heat wave in a century on Monday, with six consecutive days exceeding 40 C. The heat there buckled train and tram lines.

"Not for 100 years has it been six days or more than six days of 40 degrees or more," Matt Collopy from the South Australian Weather Bureau said.

About six people died from the heat in Melbourne, Australia's second largest city, over three days before the temperature mellowed to 31 C on Saturday, the Victoria state police deputy commissioner said.

On Friday, Melbourne recorded its third consecutive day of temperatures above 43 C for the first time since record-keeping began in 1855.

South Australia state authorities said on Saturday that the heat had probably caused some of the recent 22 sudden deaths in Melbourne. It was not yet clear how many, state Health Minister John Hill said.

Heavy air conditioner use caused a breakdown in Melbourne's electricity grid Friday night, blacking out 500,000 homes and businesses while pulling the plug on the city's electric train network.

Both Melbourne, capital of Victoria, and South Australia's capital Adelaide have experienced local blackouts in recent days as energy providers share electricity to cope with the unprecedented demand.

Ambulance services in both cities have reported increases in emergency calls because of the heat. On Friday, there were 900 calls to the ambulance service for help because of the extreme heat.

While authorities won't officially confirm some recent deaths are heat-related, extreme temperatures are taking their toll, particularly on the elderly.

One of the latest victims was a man in his 60s, living alone in the Adelaide suburb of Marion.

Police said there were no fans or air-conditioning inside his home.

The Red Cross is contacting those at risk of heat stroke and the state government is sending out thousands of text messages to warn the public to drink plenty of water and stay indoors.

In Victoria, at least 23 houses were destroyed on Friday night and Saturday by wildfires that burned 63 square kilometres of forest and farm land, a country fire service spokesman said.

Five hundred firefighters remain on high alert after fires destroyed more than two dozen homes.

Worst affected was the town of Boolarra in the Gippsland region, where some residents returned home to find that they had lost everything.