In Queensland, the fire danger is severe to extreme across three-quarters of the state, prompting authorities to send strike teams, including water bombers, to Roma, Warwick, Dalby and Toowoomba.
Fire bans have been imposed across much of Queensland, and Rural Fire Service spokesman Peter Varley said yesterday was the most dangerous faced so far this fire season.
"Under these conditions if we get a fire started it would certainly be very, very difficult to control and it will stretch our resources," he said.
Temperatures were predicted to peak at 40C or more in the interior, and in the high 30s on the coast.
Firefighters were battling outbreaks near Mt Isa, Townsville, and the Atherton Tablelands, although none late yesterday was threatening lives or homes.
Health officials warned people to take precautions in the extreme heat, and ambulance services were expecting to be kept busy by heat-related problems.
In NSW, fire crews were yesterday fighting about 30 grass and bushfires.
Victoria imposed fire bans and issued warnings as units tackled a number of large fires.
The danger across much of the state was rated as high to very high.
Similar conditions were felt across South Australia, and in Tasmania firefighters were still battling an outbreak that at one stage threatened the town of Musselroe Bay, on the island's northeast coast.
Another large fire is burning in extremely difficult terrain at South Bruny in the state's southeast.
Western Australia faced severe fire danger across large areas of its central and lower west, including Perth and the port city of Geraldton. A number of bushfires were burning in the northern regions of Gascoyne and the Pilbara.
-additional reporting AAP