The senior doctor aboard the ill-fated Ruby Princess cruise ship has told an inquiry she would not have let passengers disembark when it docked in Sydney last month.

Key points: When the vessel docked in March, dozens of passengers had coronavirus symptoms

When the vessel docked in March, dozens of passengers had coronavirus symptoms The doctor said she was "surprised" people were not told to wait for results

The doctor said she was "surprised" people were not told to wait for results An inquiry will investigate the disembarking, which resulted in 21 coronavirus fatalities

Almost 2,700 passengers were allowed to disembark the vessel on March 19, despite more than a dozen people on board displaying symptoms of, and being tested for, COVID-19.

More than 600 cases of coronavirus have since been linked to the Ruby Princess, and at least 21 deaths, making it the single biggest source of infection in Australia.

Senior doctor Ilse Von Watzdorf told the inquiry she swabbed some passengers who had presented with flu-like symptoms for COVID-19 after they tested negative for influenza.

"The times that we were in, I was trying to be sure if there was a chance of it being COVID that we would know about it and we would pick that up," she said.

The inquiry heard two asymptomatic passengers from the United States and five people with flu-like symptoms had been tested when the ship docked at Wellington in New Zealand.

It also heard more than 100 passengers had developed respiratory illness during the trip, and all those with flu-like symptoms had been confined to their cabins.

Counsel Assisting the Inquiry, Richard Beasley, asked Dr Von Watzdorf whether it was appropriate passengers were allowed to disembark.

Stay up-to-date on the coronavirus outbreak Download the ABC News app and subscribe to our range of news alerts for the latest on how the pandemic is impacting the world

"I was surprised that we were allowed to do that without waiting for the results to come through," she responded.

"If it was my decision I would've perhaps waited like the previous time."

The vessel was at the centre of a stoush between the Australian Border Force (ABF) and NSW Government, who each blamed the other for green lighting passengers to leave the vessel.

ABF's Commissioner Michael Outram said the directive to disembark came after NSW Health categorised the vessel as low-risk.

Dr Von Watzdorf agreed with Mr Beasley that there was a risk having people close together during disembarkation and allowing them to mingle with members of the community afterward.

Dr Von Wartdorf agreed it would have been wrong to say there were no signs or symptoms of COVID-19 aboard in the hours before it docked.

Under questioning from Commissioner Brett Walker SC she said she could not recall whether she said that to on-shore authorities.

"I just don't think I would have said it even at the time."

"And is that because it would have been wrong to say so, and you would like to think you wouldn't have said anything wrong — is that correct?"

"Yes."

She gave evidence via audio-visual link from the Ruby Princess, which is still docked in Port Kembla.

The ABF has ordered the ship to depart Australian waters on Thursday, but it's not clear how many crew will depart on board the ship, as at least 128 have tested positive for the virus.

The Special Commission of Inquiry will report findings in four months' time.

NSW Police were conducting a separate, criminal investigation into the circumstances surrounding the Ruby Princess's arrival in Sydney.