No immediate changes to domestic rules are on the cards following an urgent meeting of national cabinet with Prime Minister Scott Morrison on Tuesday.

A two-week delay of Australia's reopening to international students and visa holders will be used to gather more information about the variant and vaccine efficacy.

Omicron appears to be more transmissible than the Delta variant. But early indications show the new strain causes a milder form of illness.

"If that proves to be true, that's a major game change in the pandemic more broadly," Mr Morrison told reporters on Wednesday.

There was insufficient evidence showing vaccines and treatments were less effective against Omicron.

Treasurer Josh Frydenberg said Omicron was not the first and would likely not be the last mutation.

"We must hold our nerve and cool heads must prevail," he said.

UNSW epidemiologist Mary-Louise McLaws urged caution over the next fortnight while more information emerged.

"We should be on high alert because there are many things we still don't know about this particular mutation," she told the Nine Network.

"We don't know, of course, whether it gets around things like vaccine efficacy and let's hope it doesn't.

"I would advise that you just be highly cautious. You don't want to get sick over the holidays. Wear your mask when you are shopping."

Labor frontbencher Tanya Plibersek said no one wanted to see a return to widespread lockdowns.

"We need to do everything we can as a country to avoid that sort of social and economic hit. People are so tired of the lockdowns and the restrictions," she told ABC radio.

"Whatever the medical advice tells us we need to do to keep Australians safe, we should do."

In the meantime, international arrivals into NSW, Victoria and the ACT must quarantine for 72 hours.

Returning Australians from southern African countries need to quarantine for two weeks while non-citizens from that part of the world are banned.

It is likely a man who returned to Sydney from Africa has the Omicron variant, which would take NSW's tally to six.

The state on Wednesday reported 251 new infections.

NSW has increased fines for people who fail to comply with quarantine and testing requirements from $1000 to $5000.

Meanwhile, three people have been caught in the Northern Territory accused of breaking out of the Howard Springs quarantine facility early on Wednesday.

Victoria recorded 1179 additional cases and six more deaths.

The ACT had four more cases.

Australian Associated Press