Authorities believe a third case of the Omicron Covid variant is in Sydney, NSW Premier Dominic Perrottet said.
The suspected case was among the 141 who recently flew into Sydney from a southern African country.
Christmas holiday plans could be thrown into chaos as international and state border closures begin to domino and panic spreads about the emergence of the 'super-mutant' Covid variant Omicron.
NSW Health confirmed on Sunday that two travellers who touched down in Sydney on Saturday night from South Africa were infected with the new strain and two more were being tested.
South Australia and Western Australia tightened border restrictions as other premiers around the nation also start to consider harsher controls.
Scientists are in a race against time to answer three vital questions about the variant that doctors said caused 'unusual' symptoms, as Scott Morrison and Australia's business leaders call for calm.
The Australian Health Protection Principal Committee, which gave the prime minister an emergency briefing on Sunday night, are scrambling to determine how transmissible Omicron is, whether it is more severe than other strains, and if it is vaccine resistant.
Christmas holiday plans are being thrown into chaos as fears of international and state border closures domino following the emergence of the super-mutant Covid variant Omicron. Pictured: Passenger arrive at Sydney Airport
Australia slammed its international borders shut to nine African nations where the strain first appeared earlier this month.
Trade and Tourism Minister Dan Tehan said he couldn't rule out restrictions to other nations if the outbreak spreads, or more drastic measures if Omicron turned out to be dangerous.
'We've taken a cautionary approach, that's what we'll continue to take as we work through what this variant is all about and what potential impacts it may have,' he said.
'We think that's got the balance right at the moment, but there's obviously more work that needs to be done in understanding the new variant and the potential impacts it might have.'
Mr Morrison said after his meeting: 'We need to be very responsive to the further evidence that is becoming available and the expert medical advice.
'The goal is to ensure that our public hospitals and health systems are able to cope with this virus so we can live with the virus.'
Passengers undergo COVID-19 tests at the Histopath Diagnostic Specialists pre-departure area at Sydney International Airport on November 28, 2021 in Sydney, Australia
Passengers wear face masks as they arrive at the departures terminal at Sydney Domestic Airport in Sydney
NSW, Victoria, and the ACT temporarily re-imposed a 72-hour self-isolation requirement for all international arrivals.
Two other passengers who tested positive to Covid after arriving from southern Africa, one in Sydney and another at the Howard Springs facility near Darwin, are being screened to see if they also have the Omicron variant.
Victorian health authorities are also investigating whether the potential third NSW Omicron case could have infected anyone there while on a trip to Victoria.
Government sources said the Victorian Government was considering extending quarantine and reintroducing mask mandates in some settings.
New health orders could be announced as early as Monday, the sources said, and were likely to be also rolled out by state and territory leaders across the country.
NSW Jobs Minister Stuart Ayres announced on Sunday the state government was prepared to clamp down on travellers arriving from overseas.
'We will take the necessary measures, including restarting quarantine if required, to protect our community and our economy,' he said.
A government source said work was underway to restore hotel quarantine if required, but the preferred option will be home isolation unless the strain was deemed extremely severe.
WA Premier Mark McGowan on Saturday evening shut his state's borders to South Australia in a draconian effort to lock out the mutant new strain, as it allows in visitors from NSW and Victoria.
The ruling means only vaccinated South Australians can enter WA, where they must immediately go into quarantine for 14 days.
South Australia also tightened its border rules following the emergence of the Omicron variant.
All international travellers and people arriving in SA from high-risk locations in Australia will once again be required to quarantine for 14 days.
Prime minister Scott Morrison (pictured) said after his meeting: 'We need to be very responsive to the further evidence that is becoming available and the expert medical advice'
Pictured: Dr. Angelique Coetzee, the South African doctor who first alerted authorities to the presence of the COVID-19 omicron variant reported that it presents "unusual but mild" symptoms
First discovered in South Africa, Namibia, Zimbabwe, Botswana, Lesotho, Eswatini, the Seychelles, Malawi and Mozambique, the variant has since spread to several other nations across the globe.
The doctor who first raised the alarm on Omicron said patients are presenting with 'unusual' symptoms.
Dr Angelique Coetzee, who runs a private practice in the South African capital of Pretoria, said she first noticed earlier this month that Covid patients were presenting with odd symptoms.
The doctor, who has practiced for over 30 years and chairs the South African Medical Association, said none of the Omicron patients suffered a loss of taste of smell typically associated with Covid.
Instead they presented with unusual markers like intense fatigue and a rapid pulse.
'Their symptoms were so different and so mild from those I had treated before,' Dr Coetzee told The Telegraph.
She was compelled to inform South Africa's vaccine advisory board on November 18 when she treated a family of four, all of whom were suffering with intense fatigue after testing positive for Covid-19.
Australian virus expert Professor Tony Blakely it will take weeks before more is known about the strain and if it's resistant to vaccines.
'It's quite likely that this will precipitate new branches of vaccine development, there's just so many mutations on this virus it would seem most likely we will need new vaccines,' he told the Herald Sun.
That bleak possibility could send much of the globe back into lockdown but he said there could be one upside.
If Omicron is more infectious but less deadly and results in less hospitalisations, it could displace Delta as the most common form of Covid but lower the global death toll.
On the other hand: 'the worst-case scenario is it's more infectious, it's more virulent, and it's resistant to current vaccines,' Professor Blakely said.
US and Europe earlier placed six countries on the red list before Australia added another three - Malawi, Mozambique and Seychelles
The manufacturers of the Pfizer vaccine, BioNTech, said even if the new variant is resistant to vaccines it could create and ship a modified jab within 100 days.
Australian Industry Group chief executive Innes Willox worried an overreaction to the new variant could be almost as bad as the virus for businesses that were already struggling in the wake of Delta lockdowns.
'While some caution is understandable, the response to any new and inevitable variant needs to be targeted, proportionate and take into account the nearly 90 per cent of us are vaccinated and tired of lockdowns and border closures,' she told The Australian.
'We are just getting our economy back on its feet, but investment and confidence are still uncertain. An over-reaction both in terms of timing and proportionality around shutting Australia off from the world, and state from state, would be devastating.'
Australian Chamber of Commerce and Industry chief executive Andrew McKellar also warned that 'with international and domestic border closures finally reopening, business cannot afford to take a backwards step'.
'State and federal leaders must stick to the national plan to get Australia back open, and to stay open,' he said.
'Continuing to drive up the vaccination rate and encouraging everyone to get a booster shot is the best way to protect the population, rather than imposing further restrictions.'
Flight Centre founder Graham Turner is resigned to the fact the travel sector will see a huge decline in bookings over coming days and weeks.
'If the vaccines are effective against the new strain, it won't change anything in the medium term,' he said.
'It just depends on how panicked governments get, it's a political thing, they have to show they're doing something.'
University of NSW epidemiology professor Mary-Louise McLaws said 72-hour self-isolation in NSW and Victoria was not enough and advocated a return to 'Fortress Australia' immediately.
'Omicron still not fully understood. Is transmission faster, does it reduce vaccine efficacy, is it as hard to mitigate outbreaks [like] Delta?' she tweeted on Sunday.
'Until +90% vaccination coverage of total pop (not just +12yr) quarantine must be supervised for every traveller from every country. [With] testing on day-1, 4, 5.'
Dr Paul Griffin, director of infectious diseases at Mater Health in Brisbane, was far less concerned and said it was still too early to judge the risks of Omicron.
'I don't think we're back to square one. A lot of us thought this is what this virus is going to keep doing, going to keep evolving and we are going to keep finding new variants,' he told ABC.
Similar sentiments were echoed by Victorian chief health officer Brett Sutton, who conceded that it was 'impossible' to keep the strain off Australian shores but measures were already in place to reduce its spread.
Professor Sutton said he was 'very confident' vaccines would provide some level of 'cross protection' for the new variant, even if Omicron differs significantly in terms of 'how our immune system recognises it'.
'This is not back to the beginning,' he
