Health authorities have confirmed four people in Sydney are in quarantine after possibly contracting coronavirus.

Earlier this morning, the count was at one person - that person was not on board the plane that arrived from the epicentre of the deadly disease in China yesterday.

All four have been isolated in hospital for investigation of the potentially deadly disease.

NSW Health will not disclose the hospital locations of patients under investigation for privacy reasons.

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Doctors screened every passenger on the flight as it arrived from Wuhan on Thursday - but found no trace of the virus.

The disease has caused a major health alert around the world, with 17 people dead and more than 600 infected - and 18 million in lockdown.

Passengers wearing protective masks arrive at Sydney International Airport in Sydney, Thursday, January 23, 2020. Australia is working to keep out the deadly coronavirus, as a flight from the city at the centre of the outbreak arrives in Sydney. (AAP Image/Joel Carrett) NO ARCHIVING Credit: JOEL CARRETT / AAPIMAGE

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Singapore confirmed its first case, with a Wuhan man who arrived in the Asian hub on January 20 - he is currently in isolation in hospital.

On January 22, a Brisbane man who returned from China was tested to check whether he had contracted a new strain of coronavirus, but was later given the all-clear.

Meanwhile, millions of people are in lockdown in two Chinese cities at the epicentre of a coronavirus.

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Health officials fear the transmission rate will accelerate as hundreds of millions of Chinese travel at home and abroad during week-long holidays for the Lunar New Year, which begins on Saturday.

The previously unknown virus strain is believed to have emerged late last year from illegally traded wildlife at an animal market in Wuhan, a city of 11 million people in central China.