Scott Morrison tells states to hold their nerve on borders, as Paul Kelly says Omicron strain cannot be kept out

But infectious diseases experts have warned border closures could be part of a suite of measures "on the table" if the Omicron variant proves resistant to vaccines.

It came as chief medical officer Paul Kelly declared "we cannot keep the Omicron variant out forever", with a suspected case active on the NSW central coast.

National cabinet will sit on Tuesday afternoon to discuss the strain, after the federal government delayed the return of roughly 200,000 skilled migrants, international students, and humanitarian visa holders until December 15.

Prime Minister Scott Morrison said he would be urging state and territory leaders to hold their nerve by reopening internal borders by Christmas. Mr Morrison said the "temporary and cautious" pause was designed to avoid more serious disruption over the holiday season.

"We're not going back to lockdowns. None of us want that. None of us want to go back to those long quarantines and all of those sorts of issues," he said.

"[We want to] move forward into Christmas with confidence, look forward to everybody coming together for Christmas, and New Year's and the summer holidays.

"Australians are continuing to return from overseas. That hasn't changed."

A spokesperson for the NSW government said it had no plans to close their internal border.

The South Australian Australian Medical Association has called on the state government to lock out Canberrans, NSW residents, and Victorians once the variant skipped the border.

WA Premier Mark McGowan has also claimed the federal government's response to Omicron vindicated his state's hardline border stance.

Mr Morrison accepted, with varying vaccination rates and experiences of the virus, the states were in "different places".

"So they're going to have to consider those issues based on the experiences in their own states. I think that that is reasonable," he said.

Infectious diseases expert at the Australian National University Sanjaya Senanayake warned "all cards are back on the table" if the variant proved highly-infectious, resistant to vaccines, and particularly dangerous to the elderly.

Dr Senanayake said that picture would become clearer over the next month, as the variant spread through countries with quality public health surveillance.

"If we get a sense of how effective our vaccines are against Omicron that looks good, then even if it does start to circulate in the community like Delta, it shouldn't affect what we are doing too much," he said.

"But there are still a lot of ifs there."

University of South Australia biostatistics expert Adrian Esterman said states would be "very, very reluctant" to shut their borders in the lead up to Christmas, but were "almost certain" to introduce tougher testing regimes for incoming travellers.

"If we close our borders, what can we do in the meanwhile about the Omicron variant? The answer is not much," he said.

"The big thing in favour of closing borders, both internationally and state borders, is that it allows a greater percentage of people to be vaccinated."

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But with the ACT, Victoria, and NSW boasting some of the world's highest vaccination rates, Professor Esterman said they were more likely to see the return of masks and capacity limits.

"No matter what the variant we can undertake our standard and public health precautions: social distancing and wearing face masks. If I was running ACT Health ... I would reintroduce [them]," he said.

"I can't honestly see there being lockdowns in future, not at that high rate of vaccination. But I do think that we might have to reintroduce public health measures where they've been taken off."

Amid early suggestions Omicron was less severe than previous variants, some experts have suggested its spread could provide a welcome immunity top-up against more deadly strains.

Professor Esterman described that as "the ideal scenario", but warned there was currently insufficient data to know whether that was the case.

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