Considering the human and economic costs such measures impose on the community, and unless there was evidence the virus was resistant to vaccines, there would be merit in treating the highly vaccinated and neighouring jurisdictions of Victoria, NSW, and the ACT as a separate "bubble".

While it is too early to be confident of anything yet, there is no evidence so far to suggest Omicron is more vaccine resistant than previous variants. It is even being suggested it could be less lethal than what has gone before.

In light of this, snap border closures between the three most vaccinated jurisdictions could do more harm to the economic and emotional wellbeing of their residents than good. While the likes of Western Australia or South Australia may be more inclined to slam shut or stay that way, in the south-east there is an opportunity, subject to expert advice, to trust the vaccines to do their job.

The reintroduction of some of the core sanitary and prophylactic measures used over the past two years would make sense. The basics, such as reasonable crowd limits, the wearing of masks and the use of hand sanitiser, would, based on what we know now, go a long way to keeping the situation under control.

It is important to remember that at this stage the situation with Omicron is very different to what happened with Delta earlier in the year. Back then, by the time most of the world became aware of the risks there were tens of thousands of cases in the community in India and elsewhere, and it was spreading like wildfire around the globe. Australia's vaccination rate was completely inadequate at the time.

This time around, largely thanks to the good work and quick actions of South African health authorities, we have been given plenty of notice. As of Tuesday morning there were apparently fewer than 200 confirmed cases overall. Fewer than 100 of these were in South Africa. And, according to Professor Kelly, at least five of the six confirmed cases in Australia were travelling well: "They are young. They're doubly vaccinated ... and have very mild or, in fact, no disease," he said.

Professor Kelly defended the decision to push back the next stage of the reopening of Australia's international border to December 15 - which will disrupt the travel plans of tens of thousands of travellers including foreign students and skilled workers - as being made out of an "abundance of caution".

"This is a temporary pause so we can get that information we need (on vaccine resistance, virulence and lethality)," he said. "But we are committed to reopen."

It is clear, in light of these comments, that the Prime Minister's call for state and territory governments to hold their nerve and keep internal borders open for Christmas is soundly based on medical advice.

And while Mr Morrison has no direct control over state borders, he has ruled out the reintroduction of harsh lockdown measures in the states and territories.

"We're not going back to lockdowns, none of us want that," he said.

The government has the power to do this by its ability to withhold income support assistance from any jurisdiction considering such a move. No premier or chief minister would choose to go down that path if they knew their own government would be responsible for the full economic cost.

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