The dust storm hit the Hunter about 5.30am, filling the air with a thousand times the average amount of dust, conditions described as 35 times worse than those during a heavy bushfire.

The poor visibility disrupted air traffic and shipping movements, while air quality hit people with respiratory illnesses, putting pressure on an ambulance service struggling to cope with the influx of emergency calls from people with breathing difficulties.

Gale-force wind lashed the region, with gusts of up 98 kmh at Nobbys. Visibility was down to between 100 and 200 metres during the morning peak.

The Ambulance Service of NSW received 99 calls from people with breathing difficulties in the northern division between midnight and 2pm yesterday, with a spike of 20 between 7.40am and 8.40am.

There were 185 calls received in Sydney, and a total of 389 calls throughout NSW.

NSW Health issued a warning to residents in all affected areas to stay indoors, keep their doors and windows closed, and avoid vigorous exercise, indoors and outdoors.

Newcastle's air was filled with 10,650 micrograms of dust per cubic metre before noon, compared with an average of 10, Environment Department spokeswoman Liza Cassidy said.

"It's a huge amount of dust. It's just off the chart," Ms Cassidy said.

"You might get a reading of 300 with heavy bushfire smoke."

Department scientists estimated that 75,000 tonnes of dust an hour were drifting out to sea off the NSW coast.

The red dust was carried by strong wind from the western plains of NSW and inland South Australia.

Poor visibility affected in-bound and out-bound traffic at Newcastle Airport, creating long delays as aircraft were unable to land at Newcastle between 8am and 12.30pm.

Once adequate levels of visibility were restored in Newcastle in the afternoon, outbound traffic was unable to land in Brisbane and the Gold Coast as the weather system moved north, Newcastle Airport corporate affairs manager David Nye said.