About 23 people have been killed in Australia bushfires since September, PM Scott Morrison said.

Bushfires were burning dangerously out of control on Australia's east coast on Saturday, fuelled by soaring temperatures and strong winds that had firefighters battling to save lives and property.

Authorities have said conditions could be worse than New Year's Eve on Tuesday, when fires burnt massive tracts of bushland and forced thousands of residents and summer holidaymakers to seek refuge on beaches.

The government announced an unprecedented call up of army reservists to support firefighters as well other resources including a third navy ship equipped for disaster and humanitarian relief.

"Conditions beginning to deteriorate quickly on NSW southern firegrounds," the Bureau of Meteorology said in a tweet.

"Heat and wind are building which is increasing fire activity."

In South Australia, two people died on Kangaroo Island, a popular holiday spot not far off the coast, taking the national toll from this week's fires to 12. Twenty-one people remain unaccounted for in Victoria, down from 28 reported on Friday.

More than 130 fires were burning in NSW on Saturday, many out of control, and in Victoria there were evacuation recommendations for six fires, emergency warnings for 11 others and dozens more still burning.

"There are a number of large and dangerous fires burning across NSW that pose a serious threat to life," the NSW Rural Fire Service (RFS) said.

In Victoria, authorities had urged people in areas covered by a state of disaster declaration to evacuate, and said that tens of thousands of the estimated 100,000 population had left for safety.

"But there are still significant populations in those areas," said Graham Ashton, Chief Commissioner of Victoria Police. Those who stayed needed to keep monitoring emergency announcements and fire tracking apps, he said.

Following are highlights of what is happening across Australia: