Fire authorities in New South Wales say more than 130 bushfires continue to create major problems in large parts of the state, as well as the Australian Capital Territory.
They are most concerned about a fire on the New South Wales south coast as it closes in on residents and holidaymakers.
But on Wednesday the key fire-fronts are expected to be in the north of the state from the Hunter Valley up to the Queensland border.
The ABC reports that as flames carved through the night, the most serious fires burned in southern parts of the state, around Bega, Cooma, Wagga Wagga and Nowra in the Shoalhaven region. with nearly 1500 people fighting fires and thousands more on stand-by.
A total state-wide fire ban is in effect across New South Wales, with a catastrophic rating in place for the Illawarra, Shoalhaven, Southern Ranges and Riverina in the south of the state.
A "catastrophic" fire danger rating means that if fires break out they will be uncontrollable and fast-moving, so residents should leave.
The mercury skyrocketed above 40 degrees Celsius on Tuesday, accompanied by strong winds and humidity in the single digits but relief is on the way.
Cool change may help
A late southerly change hit the far south coast, with the cooler air expected to reach Sydney in the early hours of Wednesday.
Australia's Bureau of Meteorology revealed that for each of the first six days of 2013, the "national area-average" temperature had been in the top 20 hottest days on record.
It was also the first time that average national top temperatures over 39 degrees Celsius had been recorded on five consecutive days, the bureau said.
Bans on lighting fires are in place across New South Wales, Canberra, Tasmania and Victoria.
At Dean's Gap in the Shoalhaven area of the NSW south coast, a fire breached containment lines and threatened residents and Sussex Inlet as it burned more than 15 hectares.
Residents in the nearby town of Wandandian were told it was too late to flee and late at night the fire had burned within 2km of them.
The Princes Highway remains closed in both directions, as billowing smoke blocks the way out for residents who have been advised to stay and protect their homes or head to their closest evacuation centre.
In the southeast, strong winds and extremely hot weather made things difficult for crews battling a fast moving bushfire at Yarrabin near the town of Cooma - it burned more than 4000ha.
In Victoria, two homes were destroyed by a fast-moving grassfire at Chepstowe, west of Ballarat, in Victoria.
The Country Fire Authority says the fire has burnt through 225 hectares and police have been asked to evacuate nearby residents.
Two firefighters there have been treated for superficial burns to their hands.