Rural Fire Service commissioner Shane Fitzsimmons described Saturday as "awful" for firefighters, with up to 20 homes lost to the immense Gospers Mountain fire northwest of Sydney alone.

At least two more were razed near Batemans Bay on the south coast and others at Balmoral southwest of Sydney.

The destruction around Balmoral and Bargo caused by the aggressive Green Wattle Creek fire follows the loss of up to 40 homes in the area on Thursday.

However a southerly which blew through the area on Saturday night brought much-needed rain, prompting the RFS to downgrade the fire to watch and act level.

Mr Fitzsimmons said an elderly Dargan man who refused to leave, choosing to stay and defend his residence east of Lithgow, remained unaccounted for.

"There are some reports that the property is now enveloped by fire and we cannot locate that man. We're hoping that he has chosen to relocate somewhere but we just do not know," he told reporters.

At 9.30pm on Saturday there were four NSW fires burning at an emergency warning level. They included the 460,000-hectare inferno at Gospers Mountain, the 130,000-hectare Currowan blaze on the south coast and the fire at Palmers Oaky, west of Gospers Mountain.

A fire at Owendale, near Singleton, was also at emergency level, with embers starting fresh fires ahead of the main front.

Despite the rain at Green Wattle Creek, Mr Fitzsimmons said fears the southerly winds moving upstate would fan flames in new directions had come to pass.

He said it would be a number of hours before erratic fire behaviour prompted by the change gave way to milder firefighting conditions for the 3000 crews and emergency personnel in the field.

Conditions were so dry cooler conditions may also have little effect.

"On the far south coast, we are seeing a betterment behind the change but it will take a long time for behaviour to settle once that change has moved through so we've still got a long night ahead," Mr Fitzsimmons said.

NSW Ambulance chief executive Dominic Morgan said a number of firefighters suffered heat exhaustion on Saturday while battling the fires, which were having a "big impact" on demand for emergency services.

Mr Fitzsimmons added one firefighter at Lithgow had been treated for smoke inhalation and another at Gospers Mountain was struck by a car.

More than 100 extra paramedics were in the field in NSW on Saturday night.

A fire-generated thunderstorm formed over fires in the Shoalhaven region, potentially escalating danger.

"A pyro-cumulonimbus can create erratic winds and dry lightning and result in significantly faster fire spread," the RFS warned.

Similar storms could also develop over Gospers Mountain, it said, further distorting fire behaviour and direction.

The RFS said blazes could jump containment lines and threaten houses in suburban areas as winds blew embers far ahead of established fire fronts.

Before the arrival of the southerly there were 112 fires burning across the state, combined with temperatures surpassing 40C and winds gusting up to 90km/h.

Over 200 additional trucks and firefighters from NSW Fire and Rescue and the RFS were in action.

There were also significant road closures including on the Hume, Great Western and Princes highways, with holiday-makers urged to delay travel plans for 24 hours. However those roads reopened on Saturday evening.

The potentially "catastrophic" conditions across the state also meant firefighters had little time to mourn the loss of two volunteers who died in the Green Wattle Creek blaze on Thursday night.

Horsley Park Rural Fire Brigade volunteers Geoffrey Keaton, 32, and Andrew O'Dwyer, 36, died after their truck rolled.

Australian Associated Press