A thick line of dust stretching almost the entire length of NSW is headed towards Sydney, prompting warnings from health authorities.
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.
The main band was moving slowly over the Great Dividing Range by early Thursday morning.
"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.
"If possible, stay in air-conditioned premises where filtration systems can help to reduce dust particles in the air," environmental health director Richard Broome said in a statement.
"Dust may aggravate existing heart and lung conditions and cause symptoms like eye irritation and cough."
Meanwhile, snow is forecast for parts of the Snowy Mountains above 1100 metres.
"Everything's happening," Ms Pyne said.