Temperatures are soaring across NSW, with authorities concerned about dangerous bushfire conditions around the state and the potential for thick plumes of smoke to cause health problems.
The Bureau of Meteorology says "severe heatwave conditions" will build on Tuesday, bringing temperatures over 40C to many regions.
These conditions, combined with "grotty" smoke pollution, will put stress on vulnerable people in the coming days, authorities have warned.
The haze may cause breathing issues for those with heart or lung disease, while Sydneysiders have been encouraged to stay indoors and avoid exercise.
NSW Health environmental health director Dr Richard Broome said Tuesday would be "very hot and very smoky".
"It's going to be putting a lot of stress on vulnerable people, particularly elderly people who have existing heart and lung conditions," Dr Broome told reporters.
The NSW environment department says visibility across the entire Sydney basin is at "hazardous" levels on Tuesday, while air quality was very poor in southwest Sydney and hazardous in parts of northwest Sydney.
NSW Rural Fire Service Commissioner Shane Fitzsimmons says wind speeds on Tuesday won't be as high as previous dangerous bushfire days, but high temperatures and low humidity would cancel this out.
The RFS headquarters at Sydney's Olympic Park were on Tuesday briefly evacuated due to the heavy smoke in the area setting off alarms.
Firefighting crews spent Monday evening back-burning around the Gospers Mountain mega-fire on the Central Coast. That fire, as well as three other fires circling the Sydney basin, are currently at "watch and act" level.
"We're going to see this hot, dominant air mass moving across into NSW, but particularly hot conditions with really dry air is going to result in some fairly widespread severe fire danger ratings," Mr Fitzsimmons told the Seven Network.
"We're quite concerned about the potential for fire danger and fire spread today.
"We are going to see a vigorous cold front move through the state and particularly start moving through places like Sydney in the early afternoon, up through the Hunter a little later in the afternoon, into the evening."
Mr Fitzsimmons said the southerly cold front could ramp up fires surrounding Sydney by dramatically changing wind direction.
The RFS was conducting critical operations to protect homes, he said.
Nine regions are under total fire bans on Tuesday, including greater Sydney, greater Hunter and Illawarra-Shoalhaven. Severe fire danger is forecast for seven regions, including the aforementioned trio.
There were almost 85 fires burning across NSW on Monday evening with 37 of those out of control and more than 2000 firefighters in the field.
"The more up to date you can be and the more you can act in accordance with your plan or any advice or directions given by fire authorities, it could improve the prospects of survivability not just your family but also your home, your property, your livelihood," Mr Fitzsimmons said.
NSW Premier Gladys Berejiklian says the drought-stricken state could be in for a "horror summer", while Prime Minister Scott Morrison on Tuesday encouraged Australians to follow the instructions of fire authorities.
"I can reassure everyone that the nationally-coordinated effort and the specific state efforts leading the response in each of their jurisdictions has been incredibly professionally deployed," Mr Morrison told reporters.
He said the defence force was also assisting state fire authorities and thanked American, Canadian and Kiwi firefighters assisting in Australia.
Australian Associated Press