Police have launched a criminal investigation into whether the operator of the Ruby Princess downplayed potential coronavirus cases before thousands of passengers disembarked in Sydney last month.

Key points: A 17-minute triple-0 call was made from the vessel the day before passengers disembarked in Sydney

A 17-minute triple-0 call was made from the vessel the day before passengers disembarked in Sydney Police said Carnival Australia said COVID-19 was not an issue on the vessel

Police said Carnival Australia said COVID-19 was not an issue on the vessel Nearly 200 crew who are still on board are showing symptoms of COVID-19

Eleven passengers have died from COVID-19 since the vessel docked at Circular Quay on March 19 — the latest being a 78-year-old who died in Queensland earlier this afternoon.

NSW Police Commissioner Mick Fuller said there were "many unanswered questions" about the cruise, which was operated by Carnival Australia.

"There seems to be absolute discrepancies between the information provided by Carnival and what I would see as the benchmark for the laws that the Federal Government and the State Government put in place in terms of protecting Australians from cruise ships when coronavirus had started," Commissioner Fuller said.

"The only way I can get to the bottom of whether our national biosecurity laws and our state laws were broken is through a criminal investigation."

Nearly 2,700 passengers were allowed to disembark despite others on board experiencing flu-like symptoms.

Commissioner Mick Fuller said the criminal investigation would get to the bottom of whether national biosecurity and state laws were broken. ( ABC News )

More than 600 confirmed COVID-19 cases have been linked back to the cruise, which remains off the NSW coast with nearly 200 sick crew onboard.

The Commissioner said by law a vessel was only allowed to dock if the captain could assure authorities it was free from contagion.

Police told COVID-19 'not an issue' despite 17-minute emergency call

He revealed that on the day before passengers disembarked, a worker on the cruise made a 17-minute triple-0 call about two passengers who required medical assistance.

He said an ambulance supervisor was concerned about potential infection of passengers on the cruise and escalated the call to NSW Police Marine Area Command.

Commissioner Fuller said police were told by operation managers from Carnival that COVID-19 was not an issue on the ship.

"From that perspective, there are many unanswered questions," he said.

He said the homicide squad's Detective Chief Inspector Jason Dickinson would tomorrow be given materials which had been gathered during the past 48 hours.

He said the investigation would look into all agencies involved in the incident.

It is understood NSW Health Minister Brad Hazzard will be interviewed.

Carnival Australia, the Commissioner said, sent a letter to police indicating it would cooperate with the investigation and hand over any information required.

That includes emails, text messages, radio transmissions and medical correspondence between the doctor and ship's captain.

A spokesperson for Carnival Australia said it would "vigorously respond to any allegations of which there must now be full disclosure and the basis for them".

Sick crew remain onboard, some taken to hospital

Two smaller boats sail away from Ruby Princess cruise ship on Sunday. ( ABC News )

The announcement of the investigation came as emergency services staff transported crew members from the vessel to St George Hospital in south Sydney this afternoon.

Police said there were still nearly 200 crew on board the Ruby Princess with COVID-19 symptoms.

They said there were isolation plans in place and most crew members were in good health.

"We've extracted another person today who needs medical assistance and we'll continue to take people off who need medical assistance," he said.

"No-one has been denied that and no-one will be denied."

Space to play or pause, M to mute, left and right arrows to seek, up and down arrows for volume. Watch Duration: 1 minute 26 seconds 1 m 26 s Five cruise ships have been moved out of Sydney Harbour overnight.

In dramatic scenes, boats crewed by workers in hazmat suits were seen speeding to and away from the Ruby Princess, moored off Botany Bay in Sydney's south.

Three cruise ships left Sydney Harbour overnight following a mass police operation to send vessels affected by the virus home, but the Ruby Princess has remained moored off the NSW coast.

NSW Police described it as the largest peacetime maritime operation in Sydney's history.

It was completed in about 30 hours to allow the ships to leave Australian waters.