A dust storm is starting to hit Sydney, sparking an air quality warning from the state government.
The storm, which has been blown from western New South Wales, stretches about 500km.
It has blanketed regional parts of the state, reducing visibility in some areas to just metres.
Forecasters said light dust was beginning to impact on Sydney and it was expected to thicken during the day.
Bureau of Meteorology forecaster Jordan Notara said it was unclear how severe the situation would become.
"We may see some red afternoon skies," he said.
The dust storm could reach Illawarra and the Central Coast.
Mr Notara said the conditions had "the same hallmarks" of a major dust storm in Sydney in 2009.
Drought conditions have dried soil, which makes it easier for the wind to pick up dust.
The Health Department has warned those with respiratory and cardiovascular conditions, such as asthma, bronchitis and emphysema, to limit their time outside and not exercise.
"Some of the dust particles in the dust storm will be very small and can get deep into your lungs and that's why we're concerned about people's health," director of environmental health Richard Broome said.
"If possible, stay in air-conditioned premises where filtration systems can help to reduce dust particles in the air."
The Health Department said children and older adults should take care.
"Dust may aggravate existing heart and lung conditions and cause symptoms like eye irritation and cough," Dr Broome said.
The conditions are being caused by a low pressure trough and cold front that moved through South Australia this week.
That system is now over New South Wales and forecasters said the dust was travelling with the cold front through the state.
In September 2009, Sydney woke to an eerie red dawn.
Residents reported red dust covering their floors, while firefighters called the event "extraordinary", with more than 500 call-outs.
The State Emergency Service received more than 150 calls for help, mostly from people with breathing difficulties.
The dust set off smoke alarms and caused flights to be re-routed to Brisbane and Melbourne.