Temperatures had already reached 30 degrees at Glenelg in Adelaide at 7am today, with a maximum of 37 degrees expected for the city.
Other parts of South Australia could reach 42 degrees, forcing a total fire ban.
Dry and windy conditions in the state have prompted an extreme fire warning to be declared for the lower southeast region.
Stern advisories have been issued for eight districts, including Mount Lofty Ranges and Kangaroo Island.
Additional fire crews are ready to respond to any fire that occurs.
Temperatures are also forecast to skyrocket in Melbourne, with a maximum forecast of 38 degrees for the city and Geelong.
A cool change is expected to bring some late night relief.
New South Wales and the ACT
Sydney will be mostly sunny with a high temperature of 28 degrees.
Fog patches early in the morning can cloud the outer-west. Light northeast winds of up to 25 km / h are expected in the early afternoon.
The fire danger has been labeled ‘high’ for Canberra, with light winds and a maximum forecast of 33 degrees.
Melbourne’s heat wave will continue today, with the state sweating at a high of 38 degrees, before a cool change provides respite at the end of the night.
The morning will be mostly sunny, but the clouds will increase during the afternoon.
There will be a small chance of a shower in the northwest of the city later tonight with possible thunder.
Brisbane will be partly cloudy, peaking at 28 degrees.
There is a small chance of rain and southeast winds of up to 25 km / h, which will be light at night.
Adelaide will suffocate to a maximum temperature of 37 degrees as fire hazard warnings are implemented.
Winds from the northwest to the northeast of up to 40 km / h will sweep through the city in the morning, before turning from west to northwest and descending to a maximum of 35 km / h in the middle of the day.
Warm and sunny conditions are forecast for Perth, with a maximum temperature of 32 degrees.
Light east-southeast winds of up to 30 km / h will pass through the city, before turning south to southwest in the early afternoon.
Hobart will not escape the heat wave facing the southern states as the city prepares for a maximum temperature of 36 degrees.
Mostly sunny conditions are expected, with an average chance of late rains tonight.
There is the possibility of a thunderstorm with little or no rain in the late afternoon and evening.
Northwest winds of up to 40 km / h will sweep through the city during the day.
Possible probability of thunderstorm in Darwin and a maximum of 33 degrees.
There is a medium probability of rain, most likely in the late morning and afternoon.