Bushfires are threatening dozens of communities in Victoria, a southerly cool change is fanning problems in NSW, and more than a third of Kangaroo Island has burned in South Australia.
Firefighters who continued to battle fires overnight were sure more homes had been lost, but would not be able to accurately report the extent of the damage until daylight.
Since September, fires in Australia have killed at least 23 people.
More than 1300 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.
Earlier on Saturday, Prime Minister Scott Morrison announced 3000 Australian Defence Force reservists would be deployed to fire recovery across the nation to aid the ongoing crisis.
NSW has declared a week-long state of emergency. Tens of thousands of residents and holidaymakers have been told to evacuate coastal areas, where a "tourist leave zone" has been declared.
As darkness fell, several Alpine communities and towns along the Victoria-NSW border were fighting to defend from bushfires.
The blaze that devastated Corryong was pushed back towards the town, and was also impacting Walwa, Tintaldra and Towong, where officials feared properties had been lost.
It came after a day of searing temperatures in the region.
All-time heat records were broken at Albury Airport (41.6 degrees Celsius), and at Rutherglen (45.6 degrees Celsius).
Hume incident controller Paul King said those temperatures combined with south-westerly winds that "came through like a steam train" to put significant pressure on border towns.
Further south, fires were threatening Alpine communities Harrietville, Wandiligong, Bright, Mount Hotham, Falls Creek and Dinner Plain.
Authorities had been warning communities across Victoria's declared disaster zone to evacuate the area before today, amid fears the extreme weather would make bushfires unstoppable.
Army helicopters were earlier used to help dozens of people escape the fierce fire threat in the state.
Victorian authorities believe properties were lost as the fire began to move towards the border to merge with a large bushfire burning in NSW.
Officials said fire crews were battling to save Corryong's town centre, but homes on the outskirts were believed to have been lost.
On Saturday night, King said firefighters were in a better situation as temperatures began to drop, but "nowhere near any sort of containment level".
"A lot of those fires will join up and given we're two months from the fire season starting to abate, let alone finishing, we've got a long way to go," he told ABC.
Mr King said in many ways he felt it had been a "successful day", as despite the unprecedented bushfire emergency in the region, no lives were believed to have been lost.
BOM forecaster Dean Stewart said there would be a "sprinkling" of rain across Sunday and Monday for eastern Victoria, but it could only amount to 5mm in the northeast.
As much as 20mm of rain could fall in East Gippsland across the next two days, where it would come as a welcome relief to communities who have spent days fighting fires.
A number of warnings for coastal parts of East Gippsland, including Mallacoota, were downgraded to watch and act level as the cool change swept through, with temperatures dropping into the 20s.
Emergency warnings remained for blazes threatening the region's inland towns.
All of Victoria's fires could flare again next Saturday, with Police and Emergency Services Minister Lisa Neville warning the state was headed for another "spike day" of extreme weather.
NSW Premier Gladys Berejiklian warned the state was in for a "long night", with the worst fire conditions yet to come due to a late southerly wind change.
Over the course of Saturday, 13 emergency warnings were issued by the NSW Rural Fire Service (RFS), with temperatures in Penrith west of Sydney reaching a record 48.9 degrees Celsius.
Many of the emergency fires were on the South Coast between Nowra, Bega and the Snowy Mountains, where the RFS warned a fire-generated thunderstorm had formed, creating a "very dangerous situation".
Highways were closed and communities on the South Coast, the Snowy Mountains, and Sydney's doorstep were urged to take shelter after conditions deteriorated.
The Princes Highway remains closed in both directions near Jervis Bay and officials have warned drivers in some places to avoid travel that is non-essential.
The RFS has said there are reports at least 15 properties have been lost in Batlow, Talbingo, the Snowy Mountains, Bendalong and Manyana.
RFS Commissioner Shane Fitzsimmons said this number was expected to rise.
RFS Deputy Commissioner Rob Rogers said the true extent of the damage would not be known until daylight on Sunday.
"Southerly winds have come through, [and there are] a lot of reports of property loss. They won't know the full extent until tomorrow (Sunday)," he said.
"Predicted weather conditions lived up to prediction; it's been awful. We've seen fires down in the Snowy Mountains that got so big they were putting lightning out 20 to 30 kilometres ahead of the main fire."
Deputy Commissioner Rogers said the fires were heading north towards Kangaroo Valley.
"People in those areas need to have extreme caution. Fire is moving very fast ... and has been quite destructive."
As firefighters battled blazes, a 4.5 million-litre water reservoir collapsed in the town of Cooma in the early evening, reportedly causing water damage to some properties.
People across NSW were asked to cut their power use overnight due to a loss of transmission lines in the Snowy Mountains.
The loss of two substations earlier has resulted in supply issues.
Deputy Commissioner Rogers said the fire service did not anticipate worsening weather conditions in coming days, however "with the amount of fires and how dry the landscape is sometimes, you [just] need strong winds or high temperatures [and] the terrain and dryness will drive the fires".
He said even though conditions would be milder, he could not guarantee more homes would not be lost.
"I think it is supposed to get a bit windy on Thursday but nowhere near as hot, so at the end of the day we just have to take these days as they come."
The Deputy Commissioner said that more crews would be sent out to try to start the longer process of containing the fires once temperatures dropped on Saturday night.
In South Australia
More than 170,000 hectares of Kangaroo Island have been burned by two major bushfires - more than a third of the island.
The Country Fire Service (CFS) said most of the damage had been done by a fire that started near Ravine in the island's north-west, which burned about 155,000 hectares and was described as "virtually unstoppable".
Fires on Kangaroo Island have been burning for more than two weeks.
The threat on the western half of the island was downgraded to an advice message, but the Ravine fire was still out of control in the early evening on Saturday.
According to SA Tourism, there were unconfirmed reports that Kangaroo Island's Visitor and Information Centre, the KI Wilderness Retreat and Hanson Bay Wildlife Sanctuary have been damaged.
The Premier, Steven Marshall, said all buildings in the island's Flinders Chase National Park had been "very extensively" damaged.
Residents on the island were asked to limit any non-essential water usage, as the Middle River Water Treatment Plant has been affected by the fires.
Meanwhile, many properties on the island were without power.
On Saturday, damage assessment teams started calculating the loss of property and stock on the island.
One of the major sites of destruction is Southern Ocean Lodge on the island's south coast.
Dick Lang, 78, and his son Clayton, 43, were named as victims in the Kangaroo Island fires on Saturday. They died on the Playford Highway in the centre of the island.
Dick was a pioneering bush pilot, while Clayton was one of South Australia's top plastic surgeons.
Police said one of the men was found inside a car.
- ABC / BBC