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Three men are in hospital in Sydney after flying in from China, while another man in his 50s is being treated in Melbourne.
Coronavirus has killed 41 people in China and the city of Wuhan, the epicentre of the illness, remains in lockdown.
Two of the men hospitalised in Sydney flew directly from Wuhan, a 53-year-old on January 20 and a 43-year-old two days prior. The third man, aged 35, arrived from the southern city of Shenzhen on January 6.
A fourth, also aged in his 50s, was Australia's first confirmed case of the virus after he touched down in Melbourne from Guangzhou on January 19.
Only the 53-year-old man is thought to have been contagious while travelling to Australia. He flew to Sydney on China Eastern flight MU749, and authorities are obtaining details of other passengers on that flight.
NSW Chief Health Officer Kerry Chant said it was a matter of when, and not if, the virus would arrive in Sydney.
Meanwhile, passengers on China Southern Airlines flight CZ321 from Guangzhou to Melbourne on January 19 are also being contacted as a precaution.
Chinese authorities are scrambling to stop the spread of the deadly illness, restricting transport in the Hubei province including its capital Wuhan.
Coronavirus has been confirmed other countries including Thailand, Japan, South Korea, Singapore, France, Malaysia and the United States.
Experts are still learning about the virus and Australia's Chief Medical Officer Brendan Murphy says it's important people arriving from Wuhan, as well as those in close contact with them, look out for symptoms of fever, cough, sore throat, vomiting and difficulty breathing.
Australians are being told not to travel to Wuhan or China's Hubei province.