Dust storms in Australia - the world's driest inhabited continent - are not uncommon, but are usually restricted to the inland.

But during widespread drought, dust storms reach coastal areas.

Vulnerable nation

Australia is battling one of its worst droughts and weather officials say an El Nino is slowly developing in the Pacific Ocean which will mean drier conditions for eastern states.

The country is one of the most vulnerable nations to climate change, but also the world's biggest greenhouse gas emitter per capita as it relies on coal-fired power stations for the bulk of its electricity.

Some commuters experienced breathing difficulties due to the storm [Reuters]
Scientists are reluctant to directly link climate change with extreme weather events such as storms and drought, saying these fluctuate according to atmospheric conditions, but environmental groups link the two in their calls for action.

International flights were diverted from Sydney, ferries on Sydney harbour were suspended and commuter motorists warned to take care on roads as visibility was dramatically reduced.

The dust set off smoke alarms in some buildings in Sydney's central business district and brought construction to a halt.

Health authorities urged people to stay indoors, warning the dust storm was likely to continue into Thursday. More than 200 people called emergency services with breathing difficulties.

The official air quality index for the state of New South Wales recorded pollutant levels as high as 4,164 in Sydney. A hazardous level is above 200.