No one was hurt as a result of the pall that swept in overnight, bringing an eerie orange dawn to Sydney, but ambulance services reported a spike in emergency calls from people with breathing difficulties, and police warned drivers to take it easy on the roads.
Dust clouds blowing east from Australia's dry interior - parched even further by the worst drought on record - covered dozens of towns and cities in two states as strong winds snatched up tons of topsoil, threw it high into the sky and carried it hundreds of miles (kilometers).
International flights were diverted from Sydney to other cities - three from New Zealand were turned around altogether - and domestic schedules were thrown into chaos as operations at Sydney Airport were curtailed by unsafe visibility levels. Passenger ferries on the city's famous harbor were also stopped for several hours for safety reasons.
The dust over Sydney had largely cleared by midafternoon, though national carrier Qantas said severe delays would last all day because of diverted and late-running flights.
The dust was still flying further north, however, and the sky over the Queensland state capital of Brisbane was clogged with dust into the early evening.
Such thick dust is rare over Sydney, and came along with other uncommon weather conditions across the country in recent days. Hailstorms have pummeled parts of the country this week, while other parts have been hit with an early spring mini-heatwave, and wildfires.
"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.
The storms - visible as a huge brown smudge in satellite photographs of Australia on Wednesday - are the most severe since the 1940s, experts said. One was recorded traveling from southern Australia all the way to New Zealand some 1,400 miles (2,220 kilometers) away.
Officials said particle pollution in Sydney's air rose to the worst on record Wednesday, and the New South Wales state ambulance service said it had received more than 250 calls before midday from people suffering breathing problems.
People with asthma or heart or lung diseases were urged not to go outside and to keep their medicine inhalers handy.
"Keeping yourself indoors today is the main thing to do if you have any of those conditions and particularly if you're a known sensitive sufferer such as children, older adults or pregnant women," said Wayne Smith, a senior state health official.
Sydney residents coughed and hacked their way through their morning commute, rubbing grit from their eyes. Some wore masks, wrapped their faces in scarves or pressed cloths over their noses and mouths.
"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."