Police and fire authorities are investigating the cause of a blaze in the Royal National Park on Tuesday night.
The fire, which burned in the area of Farnell Avenue, Loftus, near Temptation Creek and east of Loftus Oval on the Princes Highway, was at an alert level of 'watch and act' before being downgraded to 'advice' at about 8.30pm.
Firefighters said the blaze spread quickly.
RFS Commissioner Shane Fitzsimmons said the fire in the Royal National Park at Loftus was "clearly suspicious".
On Tuesday night he said the fire had "multiple ignitions", and crews from the Rural Fire Service, Fire and Rescue NSW and NSW Parks were on the scene fighting the blaze.
Firefighters dealt with hundreds of new fires as NSW endured "catastrophic" conditions on Tuesday but authorities have warned the threat is not over as a southerly wind change sweeps across the state.
More than a million hectares of land has been consumed by bushfires that damaged or destroyed 12 more homes.
A number of firefighters were injured, but none are in a life-threatening condition, and there were no people missing as of Tuesday night, RFS Commissioner Shane Fitzsimmons said.
The bushfires, which hit hard on Friday, have claimed three lives and destroyed at least 150 homes so far.
In a fortunate turn of events, heavy smoke provided insulation from strong winds moving across the Northern Tablelands and meant several fires did not flare up as much as feared, Commissioner Fitzsimmons said.
But he warned the southerly wind change still poses a danger.
"It's very dangerous across active firegrounds," he told reporters in Sydney.
"The enormity of the task to bring these fires properly under control, to consolidate them, to get around them and mop them up is enormous."
Temperatures in the high 30s, low humidity and winds gusts beyond 70km/h coupled with drought meant the state faced "horrendous conditions", the RFS said, with 300 new fires.
While conditions are expected to ease on Wednesday, he said a forecast for severe weather into the weekend and another burst of hot air next week means "we simply aren't going to get the upper hand on all of these fires".
There were nine fires burning at emergency warning level on Tuesday night, from just north of Sydney all the way to the Queensland border.
The emergency blazes include fires in the Wollemi National Park near Lithgow, at Taree on the mid-north coast and inland from Port Macquarie.
At one point during the afternoon, there were 15 emergency fires, close to Friday's record of 17.
Firefighting efforts will be hampered as darkness falls because water-bombing aircraft can't be used overnight, Mr Fitzsimmons said, with reliance on firefighters on the ground.
A catastrophic fire danger rating - the highest possible - was current for the Greater Sydney, Greater Hunter and Illawarra-Shoalhaven regions on Tuesday.
Some 80 fires were burning at 7pm from Nowra all the way up to Woodenbong on the border. About 40 were uncontained.
The dozen or so homes impacted are in the Hunter and north coast areas, Mr Fitzsimmons said.
A fire at Turramurra on Sydney's upper north shore briefly burned at emergency level before a plane doused it with pink retardant, which was also accidentally dropped onto neighbouring homes and vehicles.
Police have established a crime scene to investigate the cause of that blaze and two males were seen being spoken to by officers at the scene.
Injuries to firefighters throughout the day included fractures, heat exhaustion, fainting and collapse, Mr Fitzsimmons said. More than 3000 firefighters and 80 aircraft were available on Tuesday to battle the blazes.
He thanked people for using the Fires Near Me app to monitor where the danger was.
The app has been opened more than 4.2 million times in recent days and the website had been visited by 1.4 million people throughout Tuesday.
Bureau of Meteorology state manager Ann Farrell says as the southerly change moves up NSW "the fires will be fanned in a different direction".
Premier Gladys Berejiklian urged residents to heed warnings and head for safety.
A week-long state of emergency has been declared in NSW with the armed forces standing by to provide support, including for search and rescue operations. Some 600 schools were closed.
The RFS received 1500 emergency calls to their operations centre since Tuesday morning.
The fire service urged people not to call triple zero except in emergencies. Residents should rather use the app to keep updated on where the fires are spreading.
With AAP and St George Sutherland Shire Leader