Premier Gladys Berejiklian declared a week-long state of emergency on Thursday as the bushfires ringing Sydney caused ash to fall in the city centre.

Some 2500 firefighters are still battling more than 100 blazes with half of them uncontained.

There were three emergency warnings current late on Thursday night for the huge Gospers Mountain blaze northwest of Sydney, the nearby Kerry Ridge fire and the Green Wattle Creek blaze southwest of the city.

The NSW Rural Fire Service says 20 homes may have been lost in and around Bargo when the Green Wattle Creek fire ripped through about midday on Thursday.

RFS deputy commissioner Rob Rogers says there are reports 40 buildings were destroyed but it's unknown how many of those were houses .

"The fire moved so quickly it just jumped rail lines, jumped roads and was impacting on property so quickly that people didn't even have the chance to react," he told ABC TV.

"We're going to have crews in the morning start going through that (damage assessment) forensically. We'll know more tomorrow (Friday)."

Three firefighters were treated for burns after their truck was overrun and enveloped by fire.

Two men - aged 36 and 56 - were flown to Concord Hospital with serious injuries while a 28-year-old woman was transported by road to Liverpool Hospital.

Mr Rogers said the firies would stay in hospital overnight but it appeared "they will make a full recovery".

Resident Ray Hicks described the moment the firestorm hit the Wollondilly Shire.

"It was horrific, just a red glow all around, and black, the wind was that horrific it just blew straight over the top of us," he told the Seven Network.

Another man told Seven he couldn't believe his home was saved: "I had my truck with water and I was pumping but I just had to leave. It was just a massive firestorm come though - it was like a wall of fire - I don't know how they (the RFS) saved the house."

The service tweeted at 7pm that "fire activity has increased" at the Gospers Mountain blaze.

"Embers are being blown ahead of the main fire," it said.

Mr Rogers later said the fire was flaring towards the township of Bell.

Conditions are set to worsen on Saturday after some respite due to a southerly change that swept up the NSW coast on Thursday evening lowering temperatures.

Earlier in the day, Sydney's CBD reached a high of 39.3C while Sydney Airport and Penrith hit 42.5C. Elsewhere, White Cliffs topped 47.1C, Mildura recorded 45.2C and Broken Hill and Wilcannia reached 45.1C.

Air quality was hazardous across Sydney and much of the state on Thursday. It's forecast to be poor in the city on Friday.

Bureau of Meteorology acting NSW manager Jane Golding said Friday will be cooler too - with a top of 26C is forecast for Sydney - but the whole state will heat up again on Saturday.

Greenpeace remains concerned that bushfires are extremely close to the Springvale coalmine near Lithgow and the Tahmoor coalmine north of Bargo.

"Coal is extremely flammable, meaning any fires could likely burn for weeks, emitting toxic fumes which will aggravate the already dangerous levels of air pollution across NSW," Greenpeace Australia's research head Dr Nikola Casule said in a statement.

A statewide total fire ban remains in place until at least midnight on Saturday.

There will be severe fire danger in the central ranges on Friday while elsewhere it will be very high or high.

Australian Associated Press