Bushfires burned 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, and authorities said the worst of conditions was yet to come.

By late afternoon, Victoria had 17 fires rated at emergency or evacuate warning levels and New South Wales had 12 rated emergency, with more than 100 others burning across the states.

"We are in for a long night and we are still to hit the worst of it," NSW Premier Gladys Berejiklian said at an afternoon briefing. "It's a very volatile situation."

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

As the NSW Rural Fire Service (RFS) updated its emergency warnings on the fires, it repeatedly delivered the same blunt advice to those who had not evacuated at-risk areas: "It is too late to leave. Seek shelter as the fire approaches."

One fire in southern NSW was generating its own thunderstorm, the RFS said, which created new dangers as lightning strikes could set off new fires.

As the fires worsened, residents used social media to post photos of the sky turning black and red from the smoke and glare of the fires, including in the Victorian town of Mallacoota, where around 1,000 people were evacuated by sea on Friday.

The federal 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.

Andy Gillham, the incident controller in the Victorian town of Bairnsdale, said the area had avoided the worst of the fires on Saturday but stressed this was an exceptional fire season.

"In a normal year, we would start to see the fire season kick off in a big way around early January and we're already up towards a million hectares of burnt country. This is a marathon event and we expect to be busy managing these fires for at least the next eight weeks," he said.

Following are highlights of what is happening across Australia:

Temperatures topped 45C (113F) in much of the Sydney metropolitan area, with Penrith recording a high of 48.9C (120F) according to the Bureau of Meteorology. Canberra, the national capital, recorded a temperature of 44C just after 4 pm, which the chief minister said was a record for the territory.

A late southerly wind change expected on Saturday will dramatically lower temperatures, but it will also bring wind gusts of 70-80 kmh (43-50 mph) that are likely to fan the strength and unpredictability of fires that have already isolated towns, with major roads and highways being closed.

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.

South Australian Premier Steven Marshall said more than 100,000 hectares of Kangaroo Island, about one quarter of its total area, had been burnt, but weather conditions have now improved after Friday's fires.

The first of thousands of residents and vacationers stranded on a beach in Mallacoota in southeastern Australia landed near Melbourne on Saturday morning after a 20-hour journey by ship. A much bigger ship, carrying about 1,000 people, is due to arrive on Saturday afternoon.

The focus on Saturday is preventing more loss of life, authorities said. National parks have been closed and people urged earlier this week to evacuate large parts of NSW's south coast and Victoria's north eastern regions, magnets for holidaymakers at the peak of Australia's summer school holidays.

National death toll in current fire season, which began in September, is 23, Prime Minister Scott Morrison said.

Morrison confirmed that his visit to India and Japan scheduled for mid-January had been postponed due to the fires.

More than 5.25 million hectares (13 million acres) of land has been burnt this fire season.

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