Australia's Chief Medical Officer Brendan Murphy says restrictions on international travel will remain in place for at least another three to four months, even if other social-distancing measures are relaxed.

Key points: Border restrictions will be some of the last to be eased

Border restrictions will be some of the last to be eased Professor Murphy said any relaxation of border rules would be "risky" for the coming months

Professor Murphy said any relaxation of border rules would be "risky" for the coming months National Cabinet will consider whether to relax small-group bans next month

Professor Murphy told a senate committee investigating Australia's response to COVID-19 that the restrictions at the border would likely be the last measure to be relaxed by the Government.

"The international situation at the moment is such that any relaxation of border measures would be very risky," he said.

"We've just recommended to the National Cabinet that we continue the very restrictive bans on Australians basically leaving the country unless there are exceptional circumstances, or anyone except Australian citizens coming back.

"The international spread of this virus is huge."

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National Cabinet has foreshadowed easing some restrictions in three weeks' time, but Professor Murphy said it was unlikely border changes would be among them.

"I wouldn't be envisaging any material changes in that three to four months," he said.

Small group restrictions could be eased in weeks

Professor Murphy shed some light on the restrictions that would be looked at when National Cabinet meets to consider relaxing some restrictions next month.

"We certainly would not be contemplating large-scale gatherings," he said.

"Certainly some relaxation of the size of small groups is possible.

"There are a range of measures that they've asked us to consider, things like community sport, some retail measures, all of those things will be in the mix.

"But we have to weigh up the public health risk versus the benefit to society and the economy."

Professor Murphy is the first government official to give evidence to the inquiry, which will not present a final report until June 2022.

Ruby Princess came as a 'surprise'

The Ruby Princess cruise ship is due to leave Australia today. ( ABC Illawarra: Justin Huntsdale )

Professor Murphy was also questioned about how the Ruby Princess cruise ship disembarked in Sydney.

He said the NSW Government had the responsibility to manage the ship's disembarkation.

He indicated the Ruby Princess was not thought to be a threat before it arrived in NSW last month.

"I was first made aware of the Ruby Princess after disembarkation had occurred," he said.

"It had only been to New Zealand and back, and New Zealand was not seen as a high-risk country.

"I think everyone was quite surprised at the fact that there turned out to have been a significant COVID outbreak on that ship. On first principles it wouldn't have normally been seen to have been a particularly high-risk vessel."

The vessel — due to leave Australian waters today — has been linked to hundreds of cases and dozens of coronavirus deaths.

Speaking earlier, the committee's chair, Labor senator Katy Gallagher, said she expected Professor Murphy would give evidence several times throughout the inquiry.

"I don't expect that we will finish with the Chief Medical Officer today, I expect that he will be a frequent visitor to our committee," she said.