Australia is looking at bringing forward booster shots and tightening international border rules in a bid to contain the new Omicron COVID-19 variant.

The Australian Technical Advisory Group on Immunisation will review the time frame for booster shots in light of the new strain that appears to be more transmissible.

Boosters are currently recommended six months after a second dose.

Federal, state and territory leaders are meeting as part of national cabinet on Tuesday afternoon to coordinate Australia's response to Omicron.

Federal cabinet's national security committee is also looking at whether the country can reopen to double-dosed visa holders, skilled workers and international students from Wednesday as scheduled.

Two fully vaccinated people who flew from southern Africa to Sydney were on Sunday confirmed to have the Omicron strain. They did not have any symptoms.

NSW authorities were looking into whether another two cases had the variant.

Separately, a man at the Northern Territory's Howard Springs quarantine facility was diagnosed with the strain.

Health Minister Greg Hunt stressed Australia was well prepared for Omicron and contracts with vaccine manufacturers covered changes for new variants.

"There are some heartening signs about what may turn out to be mild symptoms," he told reporters in Canberra on Monday.

Further changes to border rules have not been ruled out after Australia banned the arrival of non-citizens from nine southern African nations.

Victoria, NSW and the ACT already have a blanket 72-hour quarantine requirement for all international travellers.

While the new strain appeared to be more transmissible, Chief Medical Officer Paul Kelly stressed there was no definite evidence vaccines were less effective against it.

"The information from South Africa is that it has replaced Delta as the major, possibly the only, virus circulating in that country quite quickly. So it is transmitting at least as well as Delta. That seems clear," he said.

"Some reports out of South Africa are that it's mostly mild. Other information we have is that hospitalisation rates are increasing."

Prime Minister Scott Morrison defended attacks from Labor about the lack of purpose-built quarantine facilities apart from Howard Springs.

Mr Morrison pointed to a facility in Victoria under construction as well as quarantine hubs planned in Western Australia and Queensland.

Meanwhile, the South African doctor who alerted authorities to Omicron warned the rest of the world wasn't safe until Africa was vaccinated.

"As long as we don't help and assist Africa no one will sleep well at night, not even Australia, North America and not Europe," Angelique Coetzee told ABC radio.

Nearly 87 per cent of Australians aged 16 and older are fully vaccinated.

Western Australia has the country's lowest full vaccination rate of 75.51 per cent, and it's unclear what Omicron means for the state's already tentative and conditional reopening plans.

"I don't know what's going to happen with Omicron. Omicron didn't exist until three days ago," Premier Mark McGowan told the ABC.

Victoria on Monday reported 1007 new COVID-19 cases and three additional deaths.

There were 150 new infections in NSW, seven in the ACT and two in the NT.