New South Wales health authorities have been unable to contact two Australians they believe may have returned from a coronavirus-affected cruise ship in Cambodia over the weekend.

On Saturday, 22 Australians thought to have been travelling on the MS Westerdam - a cruise ship which was turned away by several Asian countries last week before Cambodia agreed to allow some of the ship's passengers and crew to disembark there - arrived in Sydney.

After Cambodian authorities allowed some passengers to leave, an elderly woman from the United States tested positive for COVID-19 in Malaysia.

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On Wednesday morning, a NSW Health spokesperson said the department had only managed to follow up with and speak to 18 of the 22 Australians.

Hours later, NSW Health said it had spoken to 20 of the 22 people and the remaining two may not have been on the cruise ship at all.

"NSW Health is liaising with the Commonwealth and the cruise ship operator to confirm whether these two people were or were not passengers on the vessel," a NSW Health statement said.

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A total of 79 Australians - 43 females and 36 males - were aboard the MS Westerdam when it docked, according to the Cambodian government.

Some remain on board the liner, some are staying at hotels in Cambodia and others have returned to their home state in Australia.

When contacted for comment, the Department of Home Affairs said it was a matter for the federal Department of Health.

The Department of Health said it was assisting states and territories to contact all passengers on the Westerdam who have returned to Australia.

"We urge passengers who have returned to Australia after disembarking the Westerdam to contact the Public Health Unit in their state for advice," a spokesperson said.

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"While it is considered very low risk as the Westerdam did not visit China, all people who were on the Westerdam should be considered close contacts, and on return to Australia should self-isolate for 14 days after leaving the vessel."

The ABC reports all Australians in Cambodia who have been tested for COVID-19 have returned negative samples.

Those still in Cambodia face uncertainty as to how and when they will get back to Australia.

Affected passengers have been stranded in Cambodia partly because there are no direct flights between Cambodia and Australia, and partly because of a Vietnamese travel ban.

Mr Sen said there is a possibility some could fly back to Australia via the Middle East.

Additional reporting by AAP.