Australian firefighters were set for a dangerous day on Saturday as fires in New South Wales (NSW) and Victoria states were expected to burn out of control due to temperatures above 40 degrees Celsius and shifting strong winds that will fan and spread the flames.

Authorities have said conditions could be worse than on Tuesday, when out-of-control fires forced thousands of residents and summer holidaymakers to seek refuge on beaches as the flames burnt massive tracts of bushland.

"It's going to be a long and difficult day for everybody," said Shane Fitzsimmons, NSW Rural Fire Services Commissioner.

More than 100 fires were burning in NSW on Saturday and more than half were not contained, Fitzsimmons said, adding winds that will shift throughout the day will spread the blazes.

Hot, dry, windy conditions make fires worse

"We know the fires we've got already, but what we need to be vigilant about today as well is the prospect of any new fires that might start under these hot, dry, windy conditions," he said.

In Victoria, where a state of disaster has been declared, there were evacuation recommendations for six fires, emergency warnings for another six, and dozens of others still burning.

"We still have those dynamic and dangerous conditions, the low humidity, the strong winds, and what underpins that, the state is tinder dry. It is really, really dry at the moment," said Andrew Crisp, Victoria's Emergency Management Commissioner.

Authorities had urged people in areas covered by the state of disaster to evacuate, and said on Saturday that tens of thousands of the estimated 100,000 population had left for safety.

There have been 10 deaths from the fires in NSW and Victoria so far this week, about half the total toll for the current fire season, and authorities have said the focus on Saturday is preventing more loss of life.

It is all about saving lives

National parks have been closed and people strongly urged earlier this week to evacuate large parts of NSW's south coast and Victoria's north eastern regions.

"Today is all about saving lives," said NSW Premier Gladys Berejiklian, who has declared a week-long state of emergency.

"All of the major road networks in NSW are still open, but we can't guarantee that beyond the next few hours. So, there are still windows for people to get out if they wish to do so."

The Australian Navy ship HMAS Sycamore delivered the first load of evacuees from the isolated town of Mallacoota on Victoria's east coast to near Melbourne, with a second vessel carrying around 900 people to dock late on Saturday.

The town was cut off on New Year's Eve by fires, and around 4,000 people were stranded on the beach. Road access is still blocked and heavy smoke has limited air access, leaving sea transport as the only reliable route out.

Since September, the fires in Australia have killed at least 20 people.

(With input from Reuters)