Dust causes 'worst ever' air pollution

Towns across far west NSW were hit with strong winds and low visibility, and dust swept up from those areas crossed through the state, blanketing Sydney.

Dr John Leys, principal research scientist with the NSW Department of Climate Change and Water, said dust storms were common in rural areas.

"Dust storms like this occur quite regularly but they rarely travel this far east and come through Sydney," said Dr Leys.

He said initial estimates showed the dust plume stretched 600km along the NSW coast from Sydney to the Queensland border on Wednesday morning, dumping up to 75,000 tonnes of dust per hour into the Tasman Sea.

Dust plume stretches 600km

"It's travelled about 1,500 kilometres to get to Sydney," Dr Leys said.

Chris Eiser, manager of atmospheric science at the department, said measurements taken in Sydney on Wednesday showed the highest level of particle concentration on record.

A normal day would see around 10 micrograms of particles per cubic metre of air and a bushfire might generate 500 micrograms.

On Wednesday, levels soared to 15,400 micrograms per cubic metre of air at one location.

"During a dust storm, that's when we get our highest levels historically and this is the highest we've measured in Sydney," Mr Eiser said.