A passenger onboard an Antarctic explorer ship stuck off the coast of Uruguay has tested positive to coronavirus, with fears of an outbreak after an additional nine people developed symptoms.

Close to a hundred Australians have been confirmed as being on board the vessel, which has been stranded in Montevideo for almost two weeks.

The MV Greg Mortimer, operated by Australian travel company Aurora Expeditions, has since ramped up discussions with the Uruguayan Ministry of Health after they were barred from disembarking due to the passengers' symptoms.

In a letter to passengers on Thursday, seen by SBS News, managing director of Aurora Expeditions, Robert Halfpenny, said the passenger with COVID-19 was in a "critically ill condition" but receiving the "best care possible" in an Uruguayan hospital.

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The ship's doctor and at least three crew members are among those that have developed fevers, he said, with three new unwell people identified in 24 hours.

"While we have enormous assistance and understanding of the situation it will not be clear cut – but we and the governments working with us know that we must find a way to get you off the ship as soon as possible," the letter read.

Family members of the 96 Australians on board say they are growing increasingly anxious about the safety of their loved ones, many of who are considered vulnerable to COVID-19.

One of them is Sydney resident Nicholas Bennett who told SBS News he was extremely concerned for his parents - Phil, 77, and Elesa, 72 - who were on a "trip of a lifetime" when the pandemic hit and are now trapped on the ship.

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"They're pretty much sitting ducks now," he said. "My father is pretty old, he's in his 70s, and they are high risk so I'm very worried about them."

The couple departed Sydney on the 11 March, two weeks before the Australian government moved to ban people from travelling overseas.

He said he hopes the government will organise to evacuate the healthy people off the ship as soon as possible.

"They're saying that there are 106 people on board who don't have the virus ... so I think it's pretty pressing that they need to do something about it," he added.

In the letter to passengers, Mr Halfpenny wrote that they would now begin comprehensive testing of healthy passengers but warned it would be difficult to "maintain the same standard of essential services on board" due to the ill health and isolation of crew.

READ MORE Evacuation flight for Australians trapped in Peru and Uruguay incoming, but many to be left behind

"Please know there is an army of people trying to bring this to an end and get you home safely," he said.

On Wednesday, a second Antarctic cruise ship, the Ocean Atlantic, with more than 120 Australians on board was allowed to dock at Montevideo port and all Australian and New Zealand travellers have since flown out of Uruguay on commercial or chartered flights.

Ocean Atlantic passengers were allowed to disembark after undergoing health checks, according to the ship's operator Chimu Adventures.

In a statement to SBS News earlier this week, the Department of Foreign Affairs and Trade (DFAT) said they were working closely with the cruise industry to provide advice to Australian passengers affected by the pandemic.

An Aurora Expeditions spokesperson said they were receiving "significant support" from DFAT and other embassies.

The Department of Foreign Affairs and Trade has been contacted for comment.

People in Australia must stay at least 1.5 metres away from others and gatherings are limited to two people unless you are with your family or household.

If you believe you may have contracted the virus, call your doctor (don’t visit) or contact the national Coronavirus Health Information Hotline on 1800 020 080. If you are struggling to breathe or experiencing a medical emergency, call 000.

SBS is committed to informing Australia’s diverse communities about the latest COVID-19 developments. News and information is available in 63 languages at sbs.com.au/coronavirus.