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Air quality plunges as NSW chokes on smoke

Smoke across parts of the Sydney basin is so thick that air pollution has been deemed 11 times worse than the typical "hazardous" level.

Medical experts, meanwhile, have deemed the air quality in Sydney among the worst on record.

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.

NSW Fire and Rescue assistant commissioner Roger Mentha said his teams had responded to more than 500 automatic fire alarms caused by smoke entering buildings.

"This amount of calls peaked between 11am and 12 midday with 154 automatic alarms, as a result there's also been over 335 triple-zero emergency calls," he told reporters in Sydney.

Mr Mentha said the volume of calls was unprecedented and had increased as the smoke cloud "descended on the city".

He's urging those whose alarms are triggered by actual fires to call triple zero and alert authorities that theirs is a real emergency.

"Resources are stretched, but they're being prioritised," he said.

The NSW environmental department on Tuesday afternoon reported 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.

NSW Health's Richard Broome says the consistently smoky conditions affecting Sydney over the past month are unprecedented.

Young people and the elderly were particularly at risk.

"The smoke here in Sydney is extremely bad today, it is some of the worst air quality we've seen," Dr Broome told reporters on Tuesday.

"We are just urging people once again to take these (conditions) seriously. If you have a heart or lung condition, it is really important to spend as much time indoors while these conditions last."

NSW Ambulance Superintendent Brent Armitage said his organisation was attending up to 100 respiratory-related call-outs per day.

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 was expected to reach Greater Sydney after 2pm.

RFS commissioner Shane Fitzsimmons says the southerly cold front could ramp up fires surrounding Sydney by dramatically changing wind direction.

Mr Fitzsimmons last week said that until a significant easterly breeze arrives the smoke in Sydney will not completely dissipate.

© AAP 2019