The Ruby Princess, a cruise ship that has become the biggest single source of coronavirus cases in Australia, docked at a port in southern New South Wales (NSW) on Monday so crew in need of urgent medical treatment could be brought ashore.

The New South Wales police earlier announced a criminal investigation into the Ruby Princess, which is owned by Carnival Corp, after its 2,700 passengers were allowed to disembark in Sydney on March 19.

More:

A Carnival Australia spokesman said on Monday the company would cooperate with the probe.

"In addition to willingly participating in the investigation, Carnival Australia will vigorously respond to any allegations of which there must now be full disclosure and the basis for them," the spokesman said in emailed comments.

The investigation will focus on communications and actions that led to the docking and disembarking of the ship's passengers to see whether national biosecurity laws or state laws were broken, NSW Police Commissioner Mick Fuller said on Sunday.

There have been at least 360 COVID-19 cases, including crew members, associated with the Ruby Princess. At least six of them are reported to have died, making up at least one-sixth of Australia's total death toll of 36 as of Monday, the Australian government said.

Australia has 5,687 confirmed cases of the disease.

Ban on cruise ships

State health authorities had classified the ship as low risk because it had sailed from Sydney to New Zealand and the Australian Border Force issued a notice allowing the passengers to travel home freely. They were required to self-isolate for 14 days.

The ship, with the remaining 1,040 crew from 50 different countries on board, docked at Port Kembla, south of Sydney, on Monday.

Two crew members were taken ashore for medical help, and more would be taken off if necessary for health reasons, Fuller said. They all have to remain in isolation for 10 days.

Cruise ships have accounted for approximately a fifth of Australia's confirmed COVID-19 cases. The government banned cruise ships from docking except for emergencies as of mid-March.

The country has seen a sharp drop off in new cases over the past week, after the country imposed tight new measures limiting public gatherings to two people, shutting pubs, restaurants and gyms, closing state borders and sending all international arrivals into hotel quarantine for 14 days.

"We are seeing some early positive signs as a result of the restrictions we're putting in place," NSW Premier Gladys Berejiklian told reporters on Monday.