Air pollution and climate change specialist Howard Bridgman, a conjoint Associate Professor at the University of Newcastle, said a combination of factors, including very dry conditions in drought-affected areas, and unstable spring weather patterns, created the dust storm.

"It's very unusual," he said. "I have been here since '77 and I have never seen a dust storm like this in the Hunter.

"Melbourne has had a couple of these, and Sydney has had one or two.

"The interesting thing is that some of this will probably go to New Zealand . . . maybe in the next two to three days."

The dust originated in the western plains of NSW and inland South Australia, Weatherwatch spokesman Don White said.

The strong wind that brought the dust was whipped up by a cold front, which passed through NSW on Tuesday, and a deep low pressure trough over Victoria, which dragged the dust east with it.

"The westerly winds which raised the dust originally were created by thunderstorms west of the ranges," Mr White said.

"There have been quite severe thunderstorms over the past 36 hours, which has created a lot of dust over an extended period.

"It's been 60 years since a dust storm reduced visibility to 100 metres."