A drop in temperatures helped firefighters battling blazes across Australia Wednesday but up to 30 wildfires were still raging out of control, destroying a handful of homes and forcing people to flee.

After facing one of the highest-risk fire days in its history on Tuesday, residents in hard-hit New South Wales state woke to much cooler conditions as a southerly change dropped temperatures significantly.

While temperatures topped 42 degrees Celsius (107.6 Fahrenheit) in Sydney Tuesday, they were forecast to peak at just 25 C a day later, while Victorian capital Melbourne was down to 20 C.

The ratings on many bushfires were downgraded with none now at the "catastrophic" level, which signifies fires will be uncontrollable, unpredictable and fast-moving, with evacuation the only safe option.

But NSW Rural Fire Service commissioner Shane Fitzsimmons warned against complacency with new fronts breaking out despite the colder weather.

"It's been a long busy night for firefighters on the ground, for incident controllers working out of those fire control centres and of course the communities affected by these fires," he told ABC television.

"We've still got a lot of fires. We've still got a lot of fire activity across NSW and as we speak we're getting reports of new fires that are developing."

Thousands of firefighters worked through the night tackling more than 140 blazes across New South Wales, Australia's most populous state, with 30 of those uncontained.

So far, only one home is believed to have been lost in the state, to a fast-moving grassfire at Jugiong near the Australian Capital Territory border.

Wildfires are a fact of life in arid Australia, where 173 people perished in the 2009 Black Saturday firestorm, the nation's worst natural disaster of modern times.

Most are ignited naturally, but in Sydney's west, three teenage boys were charged with deliberately lighting a fire in bushland on Tuesday, a day when there was a total fire ban.

They were granted strict conditional bail to appear in Parramatta Children's Court on January 29.

Victoria state has also been experiencing extreme conditions with four homes destroyed and six people treated for minor burns or smoke inhalation in a bushfire in the farming community of Carngham, which was evacuated.

Authorities said the fire was now under control.

"We have had very mild, cool conditions overnight which is a great help to the fire suppression effort," Country Fire Authority operations officer Ian Morley told reporters, with light rain in places.

Much of southern Australia has been enduring a summer heatwave and a total fire ban was still in place throughout New South Wales.

Temperatures have soared so high in recent days that the Bureau of Meteorology was forced to add new colours -- deep purple and pink -- to its charts for forecasts above the previous limit of 50 degrees Celsius.

While no deaths have been reported, police on the southern island of Tasmania continued searching burned-out properties.

Initial reports said as many as 100 people could be missing after wildfires razed more than 100 homes over the weekend. But police said there was confusion about movements during the crisis.

"We know there have been no significant injuries, which is amazing, and we are encouraged that we haven't found any human remains at this stage," Tasmanian acting police commissioner Scott Tilyard told Sky News.

In central Australia the popular tourist resort of Kings Canyon south of Alice Springs was damaged after a blaze spread from the Watarrka National Park.

Broadcaster ABC said 120 tourists were evacuated from the resort with the flames blackening many buildings, although the infrastructure remains intact.