Strong winds that have changed direction are hampering efforts by firefighters to contain bushfires in Australia's south-east.
A southerly change with powerful gusts up to 80mph (128km/h) threatened to spread huge fires raging in New South Wales (NSW), officials said.
In the neighbouring state of Victoria, army helicopters have been deployed to evacuate people trapped by the flames.
Since September, fires in Australia have killed at least 23 people.
More than 1,200 homes have been destroyed and millions of hectares of land scorched. Although much attention has centred on worst-hit NSW, every state and territory has been affected.
Early on Sunday, NSW fire commissioner Shane Fitzsimmons told Australia's 9News that "hundreds" of homes could have been lost in Saturday's fires.
Fire and Rescue NSW said four firefighters had been injured overnight. Commissioner Paul Baxter told 9News that three suffered heat exhaustion and smoke inhalation, while the fourth suffered burns to his hands while dealing with a house fire.
Australian Prime Minister Scott Morrison, who has been harshly criticised for his handling of the emergency, has announced the deployment of 3,000 reserve troops to help the fire-fighting effort.
On Saturday he was condemned again for posting an advert on Twitter showing how the government was responding to the crisis, accompanied by an upbeat backing track.
We’re putting more Defence Force boots on the ground, more planes in the sky, more ships to sea, and more trucks to roll in to support the bushfire fighting effort and recovery as part of our co-ordinated response to these terrible #bushfires pic.twitter.com/UiOeYB2jnv— Scott Morrison (@ScottMorrisonMP) 4 January 2020
Mr Morrison faced a hostile reception as he visited some of the worst-hit communities earlier this week.
NSW Premier Gladys Berejiklian said the fires had created a "very volatile situation" and "we are yet to hit the worst of it".
"We are discouraging people from moving from where they are, given the serious threats and the fact that we have so many fires at an emergency level," she was quoted by ABC as saying.
Some fire crews have been pulled out of the Snowy Mountains in the state of Victoria due to worsening conditions, The Guardian reports.
What happened on Saturday?
Skies reddened and darkened in areas of south-eastern Australia as wind gusts exacerbated the fires.
Temperatures surpassed 40C (104F) in some areas. In Penrith, west of Sydney, temperatures reached 48.9C. Some reports suggest it was for a time the hottest place on Earth.
Two naval ships rescued hundreds of people stranded on a beach after fire encircled the town of Mallacoota in Victoria.
The evacuees were taken to the port of Hastings and transferred by buses to relief centres.
"For someone who's never been in a fire, it's very, very, frightening. I'm so happy to be here," said Emily Wellington, 16, after arriving in Hastings.
Bushfires damaged two power substations in New South Wales, threatening supplies to the state including Australia's largest city, Sydney, AFP reports.
In the Batemans Bay and Moruya region in NSW, thousands of people are without power as a result of fire damage. Provider Essential Energy has warned this will continue overnight as it is not able to access the damaged area safely.
Emergency warnings were issued throughout the day urging residents to leave certain areas, while some were told it was too late to evacuate. They were instructed to find shelter.
Never thought I'd be looking at a photo of a sea of people, seeking refuge at the beach, waiting for the fire danger to pass.— Fiona Bateman (@feebateman) 4 January 2020
Will this become our 'new normal' - for future bushfires?#AustraliaBurning #bushfires #NSWfires #AustraliaOnFire #PrayForAustralia #VICfires pic.twitter.com/Z0Ipku1dYa
NSW Transport Minister and Member for Bega, Andrew Constance, has compared the South Coast fires to "an atomic bomb."
"It's indescribable the hell it's caused and the devastation it's caused," he told ABC.
Fires on Kangaroo Island in the south of Australia killed two people - a well-known pilot named Dick Lang and his son, Clayton - after a quarter of the island was ravaged by fire.
Some footage showed bushfires generating their own weather systems, including tornadoes and thunderstorms.
A fire-generated thunderstorm has formed over the Mines, Pilot Lookout and Nine Mile fires, south west of Thredbo and Ingebirah and west of Corrowong and Delegate. This is a very dangerous situation. Monitor the conditions around you and take appropriate action. #nswfires #nswrfs pic.twitter.com/2PpyCOGWGw— NSW RFS (@NSWRFS) 4 January 2020
What is being done to fight the fires?
As well as deploying the military, Mr Morrison announced A$20m (£10m) has been allocated to lease four water bomber planes. Defence force bases would provide temporary accommodation, he said.
Several organisations are raising money for relief efforts and people are giving generously.
The singer Pink has pledged to donate $500,000 (£382,000) to the Australian fire service after saying she was "totally devastated" at watching the crisis unfold.
What's the background?
The fires in Australia began in September. In addition to the fatalities, they have so far destroyed more than 1,300 homes, as well as millions of acres of bushland.
Meteorologists say a climate system in the Indian Ocean, known as the dipole, is the main driver behind the extreme heat in Australia.
However, many parts of Australia have been in drought conditions, some for years, which have made it easier for the fires to spread and grow.
Prime Minister Morrison has been criticised for his handling of the bushfires. He has faced persistent accusations of being absent, including by taking a holiday to Hawaii, and underplaying the role of climate change.
At a news conference on Friday, he said he understood that people had "suffered a great lot" and were "feeling very raw".