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Scott Morrison has assured Australians the country is not facing the same sort of challenges it endured when the coronavirus first arrived in early 2020, as a new variant emerges.

The prime minister says the emergence of Omicron is a fast-moving issue but the government will continue to be guided by the best possible medical advice.

"This is not like it was back in February and March and 2020," Mr Morrison told reporters in Canberra on Sunday.

"We now have good knowledge, good advice, the uncertainties are not like they used to be, we have good systems which have been proven."

Australia has shut its borders to nine southern African countries and states have brought in new rules for international arrivals amid concern over Omicron.

Victoria's chief health officer Brett Sutton says not enough is known about Omicron but it appears to be a very transmissible variant.

"It certainly seems to have spread very quickly in southern Africa and in the Republic of South Africa in particular," he told reporters in Melbourne.

"Numbers have been increasing evidently over a short period of time, it has become the predominant variant in that country already in a very short period of time."

Even so, Opposition Leader Anthony Albanese congratulated state governments for their swift action.

"This strain could cause a real problem," he told reporters in Melbourne.

"We know that with Delta it spread very quickly, and we need to take whatever measures are necessary."

NSW Premier Dominic Perrottet said urgent genomic sequencing is being undertaken to establish whether Omicron has already reached Sydney.

Two people who arrived from Africa overnight tested positive to the coronavirus on arrival and are now being investigated to see whether they have the Omicron variant.

In total, NSW reported 185 new infections on Sunday, but for a fourth day in a row, no new deaths were reported.

In Victoria, 1061 new cases were announced and four more virus-related deaths, while there were seven new infections in the ACT.

There were also four new cases detected in the Northern Territory, where the remote community of Lajamanu will stay in lockdown until December 11 after the virus was detected in wastewater.

There were no new infections in South Australia after reporting three on Saturday.

NSW, Victoria and the ACT have introduced a three-day home quarantine requirement for international arrivals.

At this stage it means travellers from Japan and South Korea can enter Australia without needing to quarantine from December 1 as planned, Trade Minister Dan Tehan says.

However, the Morrison government is keeping a watchful eye on developments surrounding the Omicron variant.

Mr Tehan told Sky News' Sunday Agenda program from Geneva that he believes the government hast got the balance right at this stage.

"But obviously there is more work to be done in understanding this new variant and the potential impacts it might have."

Mr Tehan travelled to Switzerland for a World Trade Organisation ministerial meeting, only to find it had been cancelled because of the clampdown on travellers from the southern African states.

© AAP 2021

Australia has shut its borders to nine southern African countries and NSW, Victoria and the ACT have brought in new rules for all international arrivals amid concern over the new Omicron variant of the coronavirus.

Urgent genomic sequencing is underway to determine whether two people who tested positive for the virus in Sydney overnight after spending time in Africa have the Omicron variant.

The two arrivals are in special health accommodation and twelve others on the flight from Doha who'd been in the region will do two weeks' mandatory quarantine.

The new variant, named Omicron by the World Health Organisation on Saturday, has been detected in South Africa, Botswana, Hong Kong, Israel, Belgium and the United Kingdom.

It has double the number of mutations as the Delta variant, which sparked a third wave of outbreaks and lockdowns in Australia this year.

Flights from South Africa, Namibia, Zimbabwe, Botswana, Lesotho, Eswatini, the Seychelles, Malawi and Mozambique will cease for two weeks under "precautionary" new federal government measures announced on Saturday.

Non-citizens who have been in those countries aren't allowed into Australia.

Australian citizens and their dependants face mandatory two-week quarantine if they have been in the region.

NSW, Victoria and the ACT will make all overseas arrivals quarantine at home for 72 hours.

People already in the state who have been in the nine countries in the past two weeks must isolate for 14 days and be immediately tested.

Anyone in those jurisdictions who have been to the nine countries in the past 14 days must get a PCR test and quarantine immediately.

In response to Omicron, South Australia extended the length of its quarantine requirement for fully-vaccinated Australian international arrivals to two weeks, and made small changes to its interstate arrival regime.

West Australian Premier Mark McGowan announced SA would be designated "low risk", introducing a quarantine requirement for international arrivals from the state.

He said it was necessary because of quarantine requirements being loosened in recent days.

Tasmania will bar entry to people who have been in southern Africa unless they have first completed two weeks of supervised quarantine on the mainland.

Commonwealth Chief Medical Officer Paul Kelly said Omicron was spreading quickly, but it wasn't clear that it caused more severe symptoms than existing strains.

It is not yet known whether existing vaccines are any less effective against the new variant than prevailing strains.

Nearly 1500 new coronavirus cases were reported nationally on Saturday, including 1252 in Victoria, 235 in NSW, seven in the ACT and one each in Queensland and the Northern Territory.

Five more deaths were recorded in Victoria.

On the latest figures, 92.2 per cent of people 16 and over in Australia have received at least their first vaccine dose and 86.55 per cent both.

© AAP 2021

The United Kingdom has announced new measures to try to slow the spread of the newly identified Omicron coronavirus variant and a top health official says there is a "reasonable chance" that vaccines could be less effective against it.

Prime Minister Boris Johnson said all arrivals would have to take a test and it was time to step up booster jabs.

"We will require anyone who enters the UK to take a PCR test by the end of the second day after their arrival and to self-isolate until they have a negative result," Johnson told a news conference.

He said those who had come into contact with people testing positive for a suspected case of Omicron would have to self-isolate for 10 days and that the government would tighten up the rules on wearing face coverings.

The program to offer booster jabs would also be stepped up, he added.

Speaking alongside Johnson, England's Chief Medical Officer Chris Whitty said there was a reasonable chance that the newly identified variant could be less easy to tackle with vaccines.

"There is a reasonable chance that at least there will be some degree of vaccine escape with this variant," Whitty said.

Earlier on Saturday, health minister Sajid Javid said two linked cases of the new Omicron variant have been detected in the UK connected to travel to southern Africa.

© RAW 2021