Hot and windy conditions are plaguing South Australia, Victoria and New South Wales, with catastrophic fire danger ratings in place for parts of the country.
An out-of-control bushfire is threatening homes in a series of small Victorian communities, but residents have been told it is too late to leave.
An emergency warning has been issued for Clunes and nearby areas north of Ballarat due to a grassfire heading south towards Dunac.
"You are in danger, act now to protect yourself. It is too late to leave. The safest option is to take shelter indoors immediately. Do not get in the car and drive. It is safer to stay where you are," the warning said on Thursday.
A fire danger rating of Code Red, signifying the worst possible conditions, was declared in Victoria and asthmatics in have been urged to take care on Thursday, as a combination of extreme pollen, strong wind, bushfires and thunderstorms could lead to serious asthma episodes.
The warning from the National Asthma Council Australia comes three years to the day after the state's "epidemic" thunderstorm asthma event that left 10 people dead and thousands seeking medical treatment.
"A high price has been paid and tragic lessons learned about prevention and awareness following Victoria’s epidemic thunderstorm asthma event," Asthma Council chief executive Siobhan Brophy said.
"People do not need to be in the immediate area of a fire to suffer from the effect of smoke on their lungs. Wherever smoke haze is visible, it is a threat to those with asthma."
Homes lost in South Australia
More than 30 people have been treated for minor injuries, and homes and sheds have been destroyed as authorities continue to battle a dangerous bushfire on South Australia's Yorke Peninsula.
The blaze is burning uncontrolled in stubble towards the towns of Edithburgh and Coobowie but the Country Fire Service says its rapid spread has been halted.
Chief officer Mark Jones says while the worst is over, fire crews still hold concerns for two towns in the fire's path.
The blaze has already burnt through about 4500 hectares.
Police commissioner Grant Stevens says at least 11 properties have been affected by the fire but the full extent of the losses was still to be determined.
He says 33 people had been treated for minor issues, some for minor burns or smoke inhalation and some who required eye washing.
Severe fire danger warning for Tasmania
Tasmanians are bracing for the state's most dangerous fire conditions this bushfire season, with dry winds and hot temperatures on the cards.
A severe fire danger rating, the third-highest of six ratings, has been issued for southern and eastern parts of the state.
Tasmania Fire Service deputy chief officer Bruce Byatt said crews are on stand-by in what is expected to be the worst conditions of the bushfire season to date.
A total fire ban stretching across the island's east, south, north and centre is in place.
Hobart is on track to hit 33C on Thursday while the temperature is set to peak in the mid-30s in other parts of the state.
A severe weather warning has been issued for damaging northwesterly winds with peak gusts up to 100km/h.
Sydney smoke haze to linger 'for days'
Meanwhile, on Thursday morning, Sydney was once again blanketed in a layer of smoke as a result of 50 bushfires still burning in the north of the state and at Gospers Mountain in the Hawkesbury region, the NSW Rural Fire Service said.
Air quality in Sydney hit hazardous levels on Thursday, according to the NSW Department of Planning, Industry and Environment, with health warnings urging children, the elderly and people with heart and lung conditions to reconsider outdoor activities.
"If you do develop symptoms and they don't get better with your normal reliever medication you should seek medical advice and in an emergency you should call triple zero," Dr Richard Broome, director of environmental health at NSW Ministry of Health, said.
The air quality across much of Sydney was hazardous for visibility on Thursday, NSW environment department readings showed.
The RFS said smoke is being mixed with dust from the state's west.
The haze and poor air quality over the city is likely to last through the weekend as bushfire smoke from the north of the state is blown south, the Bureau of Meteorology said.
"The smoke will linger around. It will go off and come back again as the wind changes but we will see these smoke haze conditions for several days," forecaster Abrar Shabren told AAP.
The new wave of smoke comes two days after paramedics treatment more than 50 people for asthma and breathing-related incidents, the Sydney Morning Herald reported.
Some 50 bushfires are still burning in NSW, 20 of which are uncontained. More than 1000 firefighters remain in the field.
There are 12 total fire bans in place across the state.
A severe fire danger rating is in place for 10 areas, including Illawarra-Shoalhaven. The entirety of the state's south is under "very high" or "severe" fire danger as hot winds blow in from South Australia.
Winds of up to 60km/h are forecast across the southern border region which could present challenges if there are new fires, Mr Fitzsimmons said.
"Another difficult day for firefighters, for people in these at-risk areas," RFS NSW Commissioner Shane Fitzsimmons told the Seven Network on Thursday.
But, after a fortnight of serious fires, he's hopeful conditions will soon improve.
"Hopefully, once we turn the corner on today there'll be an easing of conditions more broadly across the state," Mr Fitzsimmons said.
It's hoped northeasterly winds along the coast will help stop fires spreading and support firefighters in their bid to protect properties through backburning, which will continue on Thursday.
The RFS said dry lightning, something which could spark fires, is forecast for southwestern areas of the state with a southerly change on Thursday evening.
NSW Police, meanwhile, told reporters on Wednesday that legal action had been taken against 54 people since August for bushfire-related offences.