Australia will continue its path toward reopening over Christmas while health officials gather more information about the new strain.
Chief Medical Officer Paul Kelly told heads of state and territory it would be up to two weeks before there was enough information to paint a clear picture of the version of the threat.
But he said there was no evidence the vaccines were less effective.
Mr Morrison said during Question Hour on Tuesday that his government had taken “sensible precautions” in delaying this week’s plan to reopen international borders to visa holders until December 15.
About 200,000 workers and students were expected to arrive in Australia from Wednesday, but have now been delayed by at least two weeks.
The reopening of fully vaccinated travelers from Japan and South Korea has also been banned until December 15.
“We have taken sensible precautions for a two-week pause for the next steps,” Mr Morrison said.
“As we continue to encounter these new forms of anxiety, and it is one type of anxiety, we have dealt with many before.”
Mr Morrison said this was important state and region leaders were sticking to their reopening plans.
“We’re not going back into lockdown, none of us want that,” he said.
“What we did last night was defending him, by a sensible pause.”
Tasmania announced on Tuesday afternoon that it would close its borders to the vast majority of foreign arrivals.
Any traveler who has spent time in any foreign location – except New Zealand’s South Island – since November 28 will not be allowed to enter Tasmania, a post on the government’s website read.
Anyone attempting to travel to Tasmania and spending time abroad in the 14 days prior to 28 November will not be allowed to enter unless approved as an essential traveller.
Over the weekend the federal government halted flights from eight southern African countries where the variant was first detected, and brought in quarantine requirements from anyone who recently entered Australia from the region.
Victoria and New South Wales on Tuesday announced additional quarantine measures for those travelers.
Prof Kelly told reporters on Tuesday morning that health officials know the spread of the Omicron variant cannot be stopped indefinitely.
“We can’t keep this Omicron version out of Australia forever – eventually it will be here,” he said.
“The most important thing is that we now do what we can to slow that introduction and whatever measures have been introduced will help with that.”
Omicron delays border reopening for visa holders
Professor Kelly said Omicron cases in Australia have so far shown “very mild or no disease”, but the variant is continuing to be evaluated.
“We know it’s transmissible. We still don’t know about the vaccine’s effectiveness. We don’t know about the severity and there are mixed reports on that,” he said.
He said health officials are working with international partners and directly with the World Health Organization to monitor the situation “very actively”.
Omicron has prompted countries around the world to limit travel from southern Africa.
The national cabinet met on Tuesday afternoon to discuss concerns arising from the new version and existing measures to reduce its spread in Australia.
With additional reporting by you.