A visitor takes pictures in front of the Sydney Opera House during Wednesday's dust storm. (Rob Griffith/Associated Press)

A giant cloud of red dust has settled over Australia's largest city, closing the country's largest airport and prompting a spike in emergency calls.

The dust cloud settled over about a dozen towns and cities in two states on Wednesday as strong winds snatched up red topsoil in the country's dry interior and carried it hundreds of kilometres east.

The cloud of Outback grit swept into Sydney, casting the city under an orange glow, and blowing into the Pacific Ocean.

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

10 dry years

Australia's weather bureau said the dust storm is among the worst to hit the country since the 1940s.

Sydney's central business district is seen at 6:30 a.m., top, and 12:45 p.m. Wednesday as a dust storm swept through the city. (Tim Wimborne/Reuters)

"These dust storms are some of the largest in the last 70 years," said Nigel Tapper, an environmental scientist at Monash University. "Ten very dry years over inland southern Australia and very strong westerlies have conspired to produce these storms."

International flights were diverted from Sydney to other airports and some domestic flights were cancelled due to unsafe visibility. Passenger ferries around the city were also halted for safety reasons.

Ambulance services in the city reported about 250 emergency calls from people experiencing breathing difficulties. People with asthma or heart or lung diseases were urged not to go outside and to keep their medicine inhalers handy.

Officials reported that the dust set off smoke alarms in some buildings in the city's business district. The conditions also forced some construction projects to be temporarily halted.

Drivers were being warned to use caution on area roads.

Record particle pollution

The particle pollution in Sydney hit a record since levels began to be recorded in 1970 because of the dust, officials said.

Readings in the city were showing up to 15,500 micrograms of particulate matter per cubic metre, said New South Wales Environment Department atmospheric manager Chris Eiser. A normal reading on a clear day is between 10-20 micrograms, he said.

Though much of the dust in Sydney had settled by the afternoon, officials reported the grit was being blown farther north into Queensland and the capital of Brisbane was falling under its debris by early evening.

In the Outback, officials reported that some towns were under a complete blackout on Wedneday due to the dust storms. Some mines in the region have been forced to shut down until the storms settled.

The dust storms are expected to continue into Thursday, with the red dust expected to reach New Zealand about 2,220 kilometres away.

University of Queensland climatologist Samuel Marx told the Australian Broadcasting Corp. a storm of this size can move between eight and 40 million tonnes of dust.

The Bureau of Meteorology said a big cold front in New South Wales caused severe thunderstorms and gale-force winds, which whipped up the dust from inland and spread it across Australia's most populous state. Winds of more than 100 km/h also fanned bushfires in the state.

"We've got a combination of factors which have been building for 10 months already — floods, droughts and strong winds," Craig Strong, an official with DustWatch at Griffith University in Queensland, told Reuters.

With files from The Associated Press