Two Melbourne COVID patients have been found to have a variant of the virus never before been seen in the community in Australia.

The duo are linked to the Melbourne family who holidayed on the NSW South Coast last week, before reporting symptoms of the virus after returning home.

Victorian chief health officer Brett Sutton said both had the Delta variant of the virus, which has cut a deadly swathe across India and is spreading rapidly through Britain. The strain was identified through genomic testing.

“It has not been linked to any sequence of cases across Australia from hotel quarantine or anywhere else that it is not linked in Victoria or any other jurisdiction,” he said.

“It is a variant of significant concern. The fact that it is a variant different to other cases [in Melbourne] it means it is not related, in terms of transmission.”

It remains unclear how the West Melbourne family caught the Delta strain of the virus. All other infections in Victoria’s latest outbreak have been the Kappa strain, which also originated in India.

Professor Sutton said Victorian authorities were working with NSW, ACT and Commonwealth authorities to try to trace the index case. The timing of the original infection meant it might have been caught while they were on holiday at Jervis Bay.

“It is within the bounds of possibility,” Professor Sutton said.

“The average incubation time for SARS-CoV-22 is about six days. There will be individuals who go through a much more rapid transmission cycle, there will be a longer one. Five or six days puts it in NSW, Jarvis Bay territory, or indeed earlier.”

NSW Health reported another day without community COVID cases on Friday.

The West Melbourne outbreak has risen to seven cases across two families.

“This variant has been known in hotel quarantine … We are seeing it in international travellers,” Professor Sutton said.

“In terms of closely genetically sequenced related cases, we don’t have anything across Australia that matches.”

There were four more COVID infections in Victoria on Friday – three linked to the Melbourne family. The fourth is a primary close contact of an existing case and had been in isolation during their infectious period.

Victoria has 64 community virus infections, including three patients in hospital.

About 6000 close contacts remain in isolation, and 90 per cent of those have returned negative tests.

Testing commander Jeroen Weimar said there were no new infections in the City of Whittlesea cluster, which set off Victoria’s latest round of restrictions, or the outbreak related to the Port Melbourne finance company.

There have also been no further infections in aged care.

The two false positives confirmed by health authorities on Thursday night were part of the Port Melbourne outbreak. Both cases were part of the justification for Melbourne’s lockdown extension.

Victorian Acting Premier James Merlino said despite the false positives, there were no immediate plans to shorten Melbourne’s lockdown.

“Our answer on that hasn’t changed and nor should it,” Mr Merlino said earlier.

“The advice from public health remains the same.

“It (false positives or false negatives) has been a rare occurrence, but it has been a feature, when you do 100s of thousands of tests.”

On Friday morning the Health Department issued a statement, saying there were still eight cases of transmission through passing contact.

It added there are still five exposure sites where the virus had spread through people who do not know each other.

-with AAP