A dust storm that has swept across drought-stricken parts of New South Wales has shrouded Sydney's landmarks and sparked an air quality warning from the State Government.

Key points: Elderly, children and those with health complications urged to stay inside

Elderly, children and those with health complications urged to stay inside Forecasters expect the amount of dust to increase as the day progresses

Forecasters expect the amount of dust to increase as the day progresses Sydney Airport said the weather had delayed some flights

The dust storm, which stretches about 500 kilometres, on Wednesday reduced visibility to just metres in far western NSW including at Darling River and Broken Hill.

By lunchtime the impact of the dust was obvious across iconic locations like the Opera House, Sydney Harbour Bridge and Bondi Beach, while conditions in western Sydney also deteriorated.

Conditions eased throughout the afternoon, but forecasters warned the poor air quality could continue into Friday, and that people should still exercise caution and limit exposure.

The dust storm impacted several coastal areas including Sydney, the Illawarra, Hunter Valley and 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.

Sydney Airport said the weather conditions had forced it to operate with a single runway.

"International and Domestic terminals are experiencing some flight delays," the airport said in a tweet.

It wasn't as bad as 2009's dust storm - but it wasn't pretty ( Facebook: Janice Rattigan )

Passengers were told to contact their airlines for more information.

Air quality drops

Air quality deteriorated to a level considered "hazardous", according to data from the Office of Environment and Heritage.

That level was reached in north Parramatta, the Central Coast and upper Hunter region. Air quality was close to reaching hazardous levels in Sydney's eastern suburbs.

Earlier on Thursday, the Health Department 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 Dr Richard Broome said.

"If possible, stay in air-conditioned premises where filtration systems can help to reduce dust particles in the air."

Sorry, this video has expired White Cliffs blanketed in haze of red dust

The department said children and older adults should also take care.

"Dust may aggravate existing heart and lung conditions and cause symptoms like eye irritation and cough," Dr Broome said.

"Symptoms can occur for several days after dust is inhaled, so people with the chronic conditions need to be vigilant with their treatment programs."

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 NSW and forecasters said the dust was travelling with the cold front through the state.

Mr Notara said winds had picked up dry soil in drought-ravaged parts of NSW.

People have been told to stay inside as much as possible on Thursday afternoon. ( AAP: Dean Lewins )

Asthma sufferers should remain vigilant

NSW Ambulance Assistant Commissioner Toney Gately said paramedics had responded to 90 cases of asthma or breathing difficulties from midnight to 6:00am and then eight calls per hour with respiratory complaints throughout the day.

"It's too early to tell yet how much of that is related to the meteorological conditions at the moment," Mr Gately said.

He said although most of the dust storm had seemed to pass through, the NSW Ambulance Service would continue to monitor the situation.

Forecasters say the dust is travelling with the cold front moving through the state. ( AAP: Dean Lewins )

He warned those prone to respiratory problems to remain "absolutely vigilant", stay inside, drink plenty of fluids and avoid exercise.

"This warning cannot be underestimated, conditions like asthma can be deadly and to take risks in the current conditions would be very ill advised."

"Elevated dust levels can wreak havoc with the elderly and anyone else with a vulnerable respiratory system.

"Today's not the day to have the 20km run — even for healthy people. It's best to leave that strenuous effort to another day."

Deja vu for Sydneysiders

Sydney's geographical centre at Parramatta was blanketed with dust on Thursday morning. ( ABC News: Jonathan Hair )

In September 2009, Sydney woke to an eerie red dawn.

A huge dust storm had settled over the city and much of NSW after being carried east by gale-force winds days prior.

Residents reported red dust covering their floors while firefighters called the event "extraordinary" with more than 500 call-outs.

The SES received more than 150 calls for help, mostly from people with breathing difficulties.

The dust even set off smoke alarms because it simulated smoke.

The 2009 event also caused flights to be re-routed to Brisbane and Melbourne.