"Air quality in Sydney is currently classed as hazardous," he said. "It’s potentially unhealthy for everybody, but particularly for people with existing heart and lung conditions."

While the dust appears to have eased somewhat "there is still poor air quality in Sydney, so it's important that for the rest of the day people are cautious," Dr Broome said.

The unusual weather has also affected flights out of Sydney Airport, which has been reduced to single runway operations with delays and cancellations at both international and domestic terminals. Virgin and Qantas have cancelled 18 and 19 flights respectively, while Jetstar and Tiger have cancelled 6 each.

Done and dusted? Not yet

The dust cloud that came through Thursday morning is expected to continue moving through to the north-east of the state, reducing in intensity as it goes – but the weather bureau is now predicting more dust is on its way.

Strong winds are currently kicking up more dust in western NSW, which is expected to bring a second "pulse of reduced visibility and dust" to the coast by Thursday night, Mr Louis said.

While it's hard to predict the exact timing of its arrival, he said the second wave of dust is looking like coming through later on Thursday night and in the early hours of Friday morning.

"It's sort of starting up now in the west, so people in places like the Riverina will be seeing conditions deteriorate now," he said.

"We're starting to see that on the satellite image at the moment, and we expect that may move across to the eastern parts of the state this evening and overnight."