There are reports a fire in the Kosciusko National Park in the Snowy Mountains has breached containment lines.
RFS Commissioner Shane Fitzsimmons said crews would tackle the fire from the air.
“We’re about to do some more water bombing to try to halt the forward spread of that fire with the large air tanker operating out of Canberra,” he said.
“As a contingency we’re talking with campers in the likes of the Tom Groggin and Geehi camping grounds.”
The RFS is advising residents in Sydney’s north and the Central Coast that smoke in the area is coming from fires in the Hawkesbury and Hunter regions.
Cool change expected about midnight
The heat in New South Wales has been concentrated in the south-west, but will spread up and across the state, with hot air pushed from Victoria and South Australia.
Bureau of Meteorology forecaster Christopher Webb said there had been several days of extreme heat in parts of the state.
“The combination of the trough crossing and the north-westerly winds strengthening and further hot temperatures [bring] those fire dangers up into the severe range,” he said.
“A strengthening of the north-westerly wind, which is going to be hot, and dry … that’s bad for fire danger.”
Mr Webb said a cooler southerly change was expected to come up the coast from about midnight on Sunday.
The RFS said no fires may be lit in the open and all fire permits had been suspended.
South Australia sweltered through record heat on Saturday, with Port Augusta reaching 47.2 degrees Celsius.