A huge outback dust storm swept eastern Australia and blanketed Sydney today, disrupting transport, forcing people indoors and stripping thousands of tonnes of valuable farmland topsoil.

The dust blacked out the outback town of Broken Hill yesterday, forcing a zinc mine to shut down, and swept 1,167 km (725 miles) east to shroud Sydney in a red glow on Wednesday.

By noon today (Australia time) the dust storm had spread to the southern part of Australia's tropical state of Queensland.

Dust storms in Australia, the world's driest inhabited continent, are not uncommon, but are usually restricted to the inland. Occasionally, during widespread drought, dust storms reach coastal areas.

Australia is battling one of its worst droughts and weather officials say an El Nino is slowly developing in the Pacific 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.

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 green 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 New South Wales recorded pollutant levels as high as 4,164 in Sydney. A hazardous level is above 200. (http://www.environment.nsw.gov.au/AQMS/aqi.htm)

"People at risk are children, elderly, pregnant women, people with heart and lung diseases. Dust particles can increase the risk of people with these conditions becoming unwell," said Wayne Smith from the New South Wales state health department.

"EARTH, WIND AND FIRE"

The Bureau of Meteorology said a major cold front in New South Wales caused severe thunderstorms and gale-force winds, which whipped up the dust from the inland and spread it across Australia's most populous state. The 100 kilometre per hour plus winds also fanned bushfires in the state.

"This is unprecedented. We are seeing earth, wind and fire together," said Dick Whitaker from The Weather Channel.

New South Wales recently cut the state's 2009/10 wheat crop estimate by 20 percent because of dry weather.

Sydney residents told local radio that they woke to scenes from a Hollywood apocalyptic movie, while many contacted emergency services fearing a major bushfire in the city.

Karen from Sydney's inner western suburb of Dulwich Hill said she woke up to find the red dust had covered her floors and birds had been blown out of their nests.

"It did feel like Armageddon because when I was in the kitchen looking out the skylight, there was this red, red glow coming through," Karen told Australian Broadcasting Corp radio.

The blanket of dust affected most of New South Wales, the fifth biggest state or territory representing 10 percent of the island continent, and southern parts of Queensland state.

The dust storms stripped valuable topsoil from Australia's primary eastern farmlands, with some weather officials estimating some 4,000 tonnes of topsoil dumped on Sydney by dawn, with more dust blowing into the city throughout the day.

But crop analysts said the current dust storm is unlikely to have an immediate impact on wheat crops, in the country's second largest grain producing state, due to be harvested next month.