A shroud of red dust blown in from the Outback shrouded Sydney, causing chaos today at airports, keeping ferries in port and frightening residents of Australia's biggest city, dpa reported. Visibility was so low that the Opera House was obscured even from the nearby Harbour Bridge. The ochre dusting was so thick that some callers to national broadcaster ABC worried that seasonal forest fires had arrived early or that it was the end of the world. A caller identified as Mary said it was the worst dust storm to hit Sydney since 1939. Another caller who cycled her suburb to take in the moonscape said she came back "looking like a red panda." One panicked resident thought North Korea had sent over nuclear missiles. Environmental Health director Wayne Smith said that asthmatics, pregnant women and people with cardiovascular problems should stay indoors. He urged parents with children who had health problems to keep them home. The dust reduced visibility on the harbour and at Sydney airport to just a few metres, forcing cancellation of all ferry services. An airport spokesman said that 10 international flights had been diverted to Melbourne and that travellers should prepare for long delays. "Departures and arrivals delays will be experienced at both Sydney's domestic and international airports," he said. "It's going to depend how the visibility and wind goes today." Bureau of Meteorology spokeswoman Jane Golding said expected thunderstorms could compound the difficulties for travelers in the south-east of the continent. Steve, on the road out of Sydney, told the ABC that rain made for dangerous driving. "It's almost raining mud," he said. "I have a green car, and it's now an orange car. The wipers are barely able to cope with all the mud."