Sydney's sky is beginning to change colour as a thick line of dust stretching almost the entire length of NSW reaches the city.
Strong winds from a low pressure system has whipped up masses of dirt across the drought-stricken state, which is steadily heading to the coast.
A line of dust up more than 500km long can be seen from the Victorian border, through Canberra and up to Queensland.
"It's a huge system," Bureau of Meteorology duty forecaster Anita Pyne told AAP.
"We're expecting the dust to gradually increase over the next few hours, with the main band of dust to hit Sydney through the middle of the day or early afternoon. So the worst visibility is yet to occur."
It was not uncommon for inland parts of NSW to experience small-scale dust storms, but one this size was "unusual".
"It's unusual for dust events to happen on the coast because we're so much further away from that dust over the western NSW basin and we've got the Great Dividing Range in the way," she said.
It's expected the dust will keep sweeping east and may not clear the coast until Friday.
NSW Health is warning the dust will likely reduce air quality and has urged children, older people and those with respiratory conditions to take extra care.
Environmental health director Richard Broome said people should stay inside as much as possible with the air quality in Sydney already dropped to a poor level.
"Even if we can't really see the dust, it's already affecting us," he told the ABC on Thursday.
"It's a serious situation from an air quality point of view."
He said dust particle can get very deep into people's lungs and cause heart and lung conditions, asthma and emphysema to worsen.
Meanwhile, snow is forecast for parts of the Snowy Mountains above 1100 metres.
"Everything's happening," Ms Pyne said.
One talkback caller said the sky around the Blue Mountains looked as though there was an eclipse.
"It's just crazy - I've never seen it like this before," he told Sydney's 2GB radio.
"It's getting worse. You can barely see 500m past the valley."
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