Smoke across parts of the Sydney basin is so thick that air pollution has been deemed 11 times worse than the typical "hazardous" level.
Bushfires ringing the harbour city - including in the Hawkesbury region and near Warragamba Dam - have caused a thick blanket of smoke to settle in the city basin.
Ferries have been cancelled in Sydney due to the smoke haze while Sydney Trains has warned the fire alarms at train stations may be triggered.
Numerous CBD offices - including Sydney's law courts - have been evacuated due to smoke.
The Rural Fire Service headquarters at Sydney Olympic Park was also briefly evacuated due to heavy smoke in the area setting off alarms.
The NSW environmental department on Tuesday afternoon reported that the air quality index in Sydney's east and northwest was more than 11 times worse than the prescribed "hazardous" level.
The AQI provides a general indication of air pollution including visibility levels and the prevalence of particles in the air.
Sydney's southwest, meanwhile, was more than five times worse than the "hazardous" level of 200 AQI.
Those with heart and lung issues have been told to avoid all outdoor physical activity, while all people in Sydney should limit their time outdoors.
"It's going to be putting a lot of stress on vulnerable people, particularly elderly people who have existing heart and lung conditions," NSW Health's Dr Richard Broome told reporters on Monday.
Unions NSW assistant secretary Thomas Costa says no workers should be forced to be on outdoor job sites amid the haze.
"Toxicity is very, very high," Mr Costa told AAP.
"(They should) tell bosses they want to leave and we're encouraging employers to let them go."
The smoke has helped firefighters by suppressing blazes amid severe danger ratings across NSW's east but a southerly wind change is likely to partially clear the haze over the course of the afternoon.
The cold front is likely to reach Greater Sydney between 2pm and 5pm.
RFS commissioner Shane Fitzsimmons says the southerly cold front could ramp up fires surrounding Sydney by dramatically changing wind direction.
Mr Fitzsimmons said last week that until the fires are quenched or a significant easterly breeze arrives the smoke in Sydney will not completely dissipate.
Australian Associated Press