Australia's biggest city was shrouded in an eerie blanket of red dust on Wednesday as bushfires, earthquakes, wild winds and massive hail stones caused havoc in the country.

Sydney's cars and buildings turned orange as strong winds blew desert dust across the city, snarling commuter and air transport and prompting a warning for children and the elderly to stay indoors.

Residents wore face masks and covered their mouths with scarves as they travelled to work under red skies, while long delays were expected at Sydney airport after several international flights were diverted.

Elsewhere in New South Wales, hail stones "the size of cricket balls" smashed windows as thunderstorms and gale-force winds lashed the state late on Tuesday.

"We've had reports of cars with both their front and rear windscreens smashed," an official from the State Emergency Service said.

Further north, Queensland imposed fire bans across large parts of the state a day after a dozen bush blazes sprung up following a spell of unusually hot, dry weather.

Victoria state was on alert for flash floods as heavy rains fell, following a pair of minor earthquakes late on Tuesday. The 3.0- and 2.6-magnitude tremors did not cause any damage, officials said.

Police in southwestern New South Wales, bordering Victoria, reported freak conditions on Tuesday as dark red skies thick with dust cut visibility to just two to three metres (yards) in some areas.

"I've never seen anything like it in all my life, and I grew up here," a police officer at the border town of Broken Hill told national news agency AAP.

"It was darker than night time, and lasted for about half an hour. You couldn't even see the street lights."