A thick line of dust that has smothered much of NSW and Sydney has caused air quality to reach a 'hazardous' level with another wave of dust set to come.
Strong winds from a low pressure system have whipped up masses of dirt across the drought-stricken state, which headed towards the coast on Thursday.
The storm stretched more than 500km from the Victorian border, through Canberra and up towards Queensland.
Bureau of Meteorology senior meteorologist Simon Louis says more dust is expected to be pushed to the coast late Thursday night or early Friday morning.
He said the dust should not be underestimated despite its grey, hazy appearance with the majority coming from lake beds in South Australia.
"Even though it may not have looked quite so red and spectacular (as 2009) the air quality has dropped significantly," Mr Louis told reporters in Sydney.
Environmental health director Richard Broome said people should stay inside as much as possible due to the air quality between Sydney and the Hunter reaching a 'hazardous' level.
"It's important for the rest of the day people should be cautious," he told reporters in Sydney.
He said dust particles can get very deep into people's lungs and cause heart and lung conditions, asthma and emphysema to worsen.
NSW Ambulance Assistant Commissioner Tony Gately said there had been a "significant peak" in call outs with 90 people needing assistance due to asthma or breathing difficulties on Thursday morning.
Extra paramedics and call takers have been put on to meet the demand with up to eight calls coming in every hour.
"Anyone prone to respiratory distress needs to be absolutely vigilant," Mr Gately said in a statement.
"This warning cannot be underestimated, conditions like asthma can be deadly and to take risks in the current conditions would be very ill advised."
NSW Health has urged children, older people and those with respiratory conditions to take extra care.
The gusty winds which brought the dust have also affected flights at Sydney Airport with up 40 departing flights cancelled and delays across all airlines.
The Sydney Airport tweeted on Thursday afternoon both international and domestic flights have been effected as they are forced to operate on a single runway due to the weather.