Australian health officials made "unjustifiable" and "inexcusable" mistakes which allowed cruise ship passengers with Covid-19 to disembark in central Sydney, an inquiry has found.

The Carnival Corp-owned Ruby Princess was for a time Australia's biggest single source of infection, with more than 600 cases and over 20 deaths directly linked to those passengers.

Some 2,700 passengers, 120 of whom were feeling unwell, were allowed to leave the ship on 19 March last, helping spread the virus across the country and internationally.

Health officials in New South Wales - the Australian state where Sydney is located - failed to ensure that sick passengers were isolated in their cabins, a report by the inquiry concluded.

The health service also failed get quick test results for unwell passengers before they disembarked.

"The delay in obtaining test results for the swabs taken from the Ruby Princess on the morning of 19 March is inexcusable. Those swabs should have been tested immediately," the inquiry, led by lawyer Bret Walker, said in the report.

NSW Health directed media inquiries to the state premier,Gladys Berejiklian, who said in a statement she would read the report over the weekend before responding.

The 315-page report did not make formal recommendations and stopped short of calling for government resignations.

Carnival Corp said the report confirmed that none of its employees misled Australian authorities, which was "of great importance to us because it goes to the integrity of our people".

The Ruby Princess had completed a Sydney-New Zealand roundtrip when it docked in Australia. Around two thirds of the passengers were Australian.