Fire officials said a statewide fire ban was posted for New South Wales, with a "catastrophic" rating in Illawarra, Shoalhaven, Southern Ranges and Riverina, and an "extreme" danger level for other areas, the Australian Broadcasting Corp. reported.
Total fire bans also were in effect across Tasmania, the southern island state seared by fire in recent days.
A "catastrophic" warning includes the risk of significant loss of life and the destruction of many homes, the NSW Rural Fire Service said.
So far, there have been no reported deaths, but Tasmanian officials said about 100 people were unaccounted-for.
Residents have been warned to remain on alert as temperatures climb to a predicted high of 109 degrees F in Sydney, CNN said.
"I cannot say it more plainly: the risk is real and potentially deadly. People need to act now," fire Commissioner Shane Fitzsimmons said.
The Australian government Tuesday announced the state fire service would have access to Defense Force bases, fuel and personnel.
Australian Prime Minister Julia Gillard also warned Tuesday was a "dangerous" day.
"The word 'catastrophic' is being used for good reason," Gillard told Seven Network. "It is very important that people keep themselves safe, that they listen to local authorities and local warnings."
In Tasmania, crews searched for about 100 people who still haven't contacted friends or family, police said.
"It's vitally important that all people who were in the area at the time, and are OK, self-register their details with the National Registration and Inquiry Service operated by the Red Cross," Acting Deputy Commissioner Donna Adams said.