The authorities said on Friday that thousands of homes in Melbourne and Adelaide had experienced blackouts since Thursday due to power drains by air conditioners.
Peter Batchelor, Victoria's energy minister, said "to prevent any further rolling blackouts, we're asking people to be very careful with their use of electricity today".
Richard Carlyon, of the weather bureau, said Melbourne was also experiencing its driest period since 1965 with 27 days without rain.
Geoff Conway, the Country Fire Authority deputy chief fire officer, said three rural towns in Melbourne were under threat from wildfires spreading quickly in the heat.
The ambulance service in Victoria state, where Melbourne is located, said more crews were on standby on Friday to help people affected by the heat, a day after 1,305 emergency cases were logged.
Train tracks buckle
The other major city on the south coast, Adelaide, is expected to match its longest heat wave in a century by early next week, with six consecutive days exceeding 40 Celsius.
Adelaide commuters have been warned to expect delays because the heat had caused train and tram lines to buckle.
Temperatures in Adelaide this week reached a high of 45.7 Celsius on Wednesday, a temperature not seen in 70 years.
That was followed by an overnight low of 33.9 degrees Celsius, the warmest on record.