Prime Minister Scott Morrison says "aggressive" questioning that reduced a NSW Health employee to tears at the Ruby Princess inquiry was "out of line".

Epidemiologist Kelly-Anne Ressler, who had direct contact with the cruise ship before it docked in Sydney, was yesterday questioned for hours on the department's precautionary procedures.

At one stage she broke down while admitting it was "unsatisfactory" to have tested more people on board for influenza than COVID-19, despite signs of illness.

"If we could do it again it would be very different," Ms Ressler said through tears.

Commissioner Bret Walker SC asked Ms Ressler why he should not conclude there had been a "reprehensible failure" from the state's health department.

Mr Morrison today told radio station 2GB he found the evidence distressing and public health workers were "doing their best".

"They have been working day and night for months and months and months," Mr Morrison said.

"I know we've got to get to the truth on this sort of stuff, but my first blush on that one — and that's not to call into question the independence of the royal commission or anything like that — but I found that a bit out of line."

Mr Morrison said Ms Ressler should be thanked for her work.

"To see her reduced to that, under that sort of aggressive line of questioning, you've got to get the balance right on this one and I would hope Mr Walker would reflect on that."

The inquiry is probing the decisions made around the disembarkation of the ship's 2,700 passengers in March despite the fact test results for COVID-19 were still yet to be returned.

Carnival Australia port agent Dobrila Tokovic today gave evidence that she found out the day before the ship arrived NSW Health officials would not board the vessel when it docked.

She said that was "in some ways" surprising.

"Just from my own experience, having 100 people unwell for a duration of a cruise would not stand out as a significant number," she said.

Counsel assisting the inquiry Richard Beasley SC replied: "But that experience is almost entirely in a pre-COVID world."

The Ruby Princess has been linked to nearly 700 coronavirus cases and at least 21 deaths and is Australia's largest single source of infection.

The ship was given a rating of "low risk" by NSW Health based on a log of onboard illness that was 18 hours old by the time the vessel docked, the inquiry has heard.

Had the log been updated, the percentage of sick passengers and crew would have pushed it into a higher risk category and triggered more NSW Health intervention, including onboard assessments.

The inquiry is due to report by mid-August.

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

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