Authorities in Victoria on Friday issued an alert as a new Covid-19 variant was detected in the Australian state's latest outbreak and the source of the strain is still unidentified.

The B.1.6172 coronavirus variant, also known as Delta, has been detected in two members of a family of four in West Melbourne, reports Xinhua news agency.

However, the health authorities are still in search of how the persons were infected with the variant, since other cases were related to the Kappa strain.

"It is a very significant concern," Victoria Chief Health Officer Brett Sutton said.

"It has not been linked to any sequence cases across Australia from hotel quarantine or anywhere else."

Sutton told the press on Friday that the Delta variant had very high transmission potential, and there isn't much information about severity of illness with this variant, although there were some anecdotal reports of greater severity in children.

The family also travelled to Jervis Bay in the neighbouring state of New South Wales (NSW).

Currently Victorian health authorities are working with their counterparts in NSW, Australian Capital Territory and the federal.

"We are examining who is the likely index case in this family, and therefore trying to trace back, where this variant has been picked up," Sutton said.

In addition, the health authorities are chasing down all those primary close contacts of that family.

"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 otherwise," Sutton said.

The state recorded four new local cases on Friday, the first day of its extended lockdown, from 494,39 tests results received in the past 24 hours and another two cases in returned travellers.