Hundreds of homes are feared destroyed and the NSW premier believes it will be a miracle if no lives are lost in the most grave bushfire crisis to hit the state in a decade.
While the extent of the devastation was unclear on Thursday night, one of the worst-hit areas was Springwood, in the Blue Mountains, where up to 30 homes were known to be lost.
But when the ashes settle, the number of destroyed or damaged properties across the state is expected to be much worse.
Elsewhere, thousands of firefighters were struggling against around 100 blazes across the state - on the Central Coast and further north, the Southern Highlands and the south coast.
It was too soon to estimate how many properties had been lost, but Rural Fire Service Commissioner Shane Fitzsimmons predicted: "we'll be counting properties in the dozens, if not the hundreds."
Premier Barry O'Farrell and Mr Fitzsimmons told reporters the public should brace for widespread destruction.
"It will take some days until we see the end of these fires," Mr O'Farrell warned.
"I suspect that if we get through that without the loss of life we should thank God for miracles."
Mr Fitzsimmons said firefighters faced the worst of conditions.
"This is as bad as it gets," he said.
Schools at the Blue Mountains were also drawn into the drama and scores of Blue Mountains residents sought refuge at evacuation centres on Thursday night, including the Springwood Sports Club and Springwood Country Club.
While St Columba's students were kept in their school, St Thomas Aquinas School was evacuated.
About 600 kids from several schools were bussed to the Springwood Sports Club on Thursday evening.
Many have since left with parents but a lot won't be able to return to their homes, the ABC reports.
A 2000-hectare bushfire at Muswellbrook that's out of control was at 8.50pm (AEDT) threatening properties.
Residents have been advised to leave the area after the "watch and act" alert was upgraded to an "emergency warning."
"Firefighters are undertaking property protection under difficult, dangerous and erratic weather conditions," the RFS says on its website.
A fire at North Doyalson has been downgraded from an "emergency warning" alert level to a "watch and act."
RFS Deputy Commissioner Rob Rogers said it was one of the worst days he'd seen.
"It's probably the most serious fire risk we've faced since the early 2000s," he said.
For most of the day there were six fires at "emergency warning" level, meaning homes were at risk and residents were being asked to consider fleeing.
Before 6pm, a cool change brought temperatures down from the mid-30s to the mid-teens.
But it also created new chaos, swinging fire fronts around and pushing blazes into new areas.
There were unconfirmed reports of properties being lost at North Doyalson, on the Central Coast; at Lithgow; at Yanderra and Balmoral, in the Southern Highlands; and in Port Stephens, where a fire has forced the closure of Newcastle Airport.
At least two firefighters were injured, with one man sent to Sydney's Concord Hospital with burns to his face.
The fires created traffic chaos around Sydney, with a 20km queue on the Hume Highway for city-bound traffic.
All northbound lanes on the highway were reopened about 9.45pm (AEST), the Transport Management Centre said.
Only one southbound lane is opened between Wilton and Mittagong and the TMC is warning drivers to expect significant delays.
The Insurance Council of Australia (ICA) has formally declared a "catastrophe" for affected areas.ICA CEO Rob Whelan said the group expects to have a better idea of the damage by the weekend, but added that insurers were ready to take claims.