An Australian patient with the deadly coronavirus returned from China more than three weeks ago before presenting to health authorities, it has emerged.
As the world scrambles to prevent the fatal respiratory illness from spreading, Australia's chief medical officer warned more cases were likely.
It comes as China's president Xi Jinping convened an emergency meeting, warning that the virus spread was accelerating as the death toll jumped from 26 to 41 in a day.
Three men from NSW and one Victorian male are in isolation after coming forward with symptoms, with the Victorian man contracting pneumonia.
The NSW men had travelled from China to Australia on January 6, 19 and 20, potentially infecting more people on flights and in the community.
Authorities are attempting to contact passengers who shared planes with the four victims.
Australia's chief medical officer Professor Brendan Murphy said he did not believe the men were contagious during their flights because they did not show symptoms at that time.
Evidence of the sickness did not become apparent until after each of them arrived in Sydney.
However experts are still learning about the virus and Professor Murphy urged people arriving from Wuhan, as well as those in close contact with them, to look out for symptoms of fever, cough, sore throat, vomiting and difficulty breathing.
"There are potentially others like this person who have travelled to Australia who were well when they arrived who may develop the disease," said Professor Murphy.
More than 1300 people have been infected globally, most of them in China where the virus first emerged from a seafood market in Wuhan that was illegally selling wildlife.
Coronavirus has been confirmed other countries including Thailand, Japan, South Korea, Singapore, France, Malaysia and the United States.
The Australian cases
Two of the Australian cases 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.
Meanwhile, passengers on China Southern Airlines flight CZ321 from Guangzhou to Melbourne on January 19 are also being contacted as a precaution.
The Melbourne coronavirus patient spent two weeks in Wuhan before coming to Australia on Sunday, January 19 but did not contact the GP until Thursday 23 January after developing symptoms.
Australians are being told not to travel to Wuhan or China's Hubei province.
Hong Kong emergency
Hong Kong has declared a virus 'emergency', restricting links to mainland China, where the death toll from the coronavirus has jumped to 41 from 26 a day earlier.
The US is arranging a charter flight on Sunday to bring its citizens and diplomats back from Wuhan, the central Chinese city that is the epicentre of the outbreak, the Wall Street Journal reported.
France is also planning to evacuate French nationals trapped by the Chinese government's lockdown in Wuhan, the South China Morning Post reported.
In Hong Kong, with five confirmed cases, the city's leader Carrie Lam said flights and high speed rail trips between the city and Wuhan would be halted.
Schools in Hong Kong that are currently on Lunar New Year holidays will remain closed until February 17 and education authorities have asked universities to extend leave for students.
State-run China Global Television Network reported in a tweet on Saturday that a doctor who had been treating patients in Wuhan, 62-year-old Liang Wudong, had died from the virus.
US coffee chain Starbucks said it was closing all its outlets in Hubei province for the week-long Lunar New Year holiday, following a similar move by McDonald's in five Hubei cities.