ACT Health Minister Rachel Stephen-Smith said there would not yet be changes to travel advice for people in the ACT, despite the mystery surrounding the origin of the cases.

Four Victorians who tested positive for COVID-19 after travelling through popular NSW holiday destinations are not linked to the wider outbreak in that state, and it is unclear where they caught the virus, Victorian health authorities announced on Friday morning.

Victorian Chief Health Officer Brett Sutton said the sequencing had identified the cases were the Delta variant of COVID-19, which is the dominant strain in India and increasingly in the United Kingdom.

"It is a variant of significant concern," he said.

Areas of NSW and the ACT were put on alert this week after a family of four tested positive for the virus after camping in Jervis Bay and visiting popular stopovers along the Hume Highway in Goulburn and Gundagai.

On Friday morning Victorian authorities revealed those cases weren't genomically linked to other cases within the state, leading to a wider investigation to find out if the cases are linked to a different leak out of hotel quarantine.

"The fact that it is a variant different to other cases it means it is not related, in terms of transmission, with these cases. It has not been linked to any sequence cases across Australia from hotel quarantine or anywhere else that it is not linked in Victoria or any other jurisdiction," Professor Sutton said.

Professor Sutton said the Victorians were working with NSW, the ACT and the Commonwealth due to the combination of responsibility for the area. The fact that these cases were unrelated to Victoria's other cases was raised at a phone hook up between the two states, the ACT and the Commonwealth on Friday morning.

"In terms of the ongoing sequencing, we will try to look at all other sequences, re-sequenced to the fullest extent possible across Australia to see if there are any potential linkages to known cases and that includes those who have come through formal quarantine but also anyone else, maritime, airline, diplomatic and others.

"It is a concern that it is not linked to other cases but we are chasing down all those primary case contacts for that family and looking into where it might have been acquired."

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Ms Stephen-Smith said the ACT was working closely with its counterparts.

"Obviously the ACT is located within NSW and given the amount of people movement between the ACT and our surroudning region, we'll continue to work very closely with NSW as we have done throughout this entire pandemic," Ms Stephen-Smith said.

"If we need to respond with additional travel and health advice and measures for our community, we will advise that as quickly as we possibly can. At this time, however, our current travel restrictions remain in place."

There are 62 people in the ACT in quarantine who have been identified as close contacts from the exposure locations in NSW, while seven close contacts from Victorian sites also remain in quarantine in the ACT.

There were about 580 people in the ACT subject to stay-at-home orders as a result of the Victorian outbreak, Ms Stephen-Smith said.

Professor Sutton said all avenues were being explored to find out the index case in the cluster, which had grown to seven with three more cases connected to the family confirmed in Victoria overnight. It's believed the virus spread between two grade five students at an inner-suburban primary school in Melbourne.

Professor Sutton said that while there had been cases of the Delta variant in hotel quarantine in Australia "in terms of closely genetically sequenced related cases, we don't have anything across Australia that matches" these cases.

Health authorities across Victoria, NSW and the ACT were investigating their databases to find links with other cases that had been detected.

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Following the National Cabinet meeting on Friday, Chief Medical Officer Paul Kelly said authorities were meeting to discuss the cases.

"That is another mystery for us to work on and AHPPC is meeting now to discuss that," he said.

Asked if the AstraZeneca vaccine was less effective against the Delta variant, Professor Kelly said that was still to be seen.

"There has been some work in the UK which suggest there may be a lower efficacy of AstraZeneca but it is not zero and that is preliminary laboratory work and we have to wait for the real world experience which only comes when we have large outbreaks and I'm sure we will not be having those in Australia," he said.

This week the World Health Organisation announced a new naming system for different variants of COVID-19, moving away from naming variants from the country where they were first found. Both the Kappa and Delta variants were first found in India and have been referred to as "Indian variants" of the disease. The majority of COVID-19 cases in Victoria, linked to the virus leaking out of hotel quarantine in South Australia, are the Kappa variant.

The family that travelled to Jervis Bay have the Delta variant of the disease, making them unrelated to the wider outbreak.

- with Jasper Lindell

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