Victoria has recorded five new local cases of coronavirus as authorities scramble to find the source of an alarming new strain of coronavirus.

There was one new case in hotel quarantine which is understood to have been infected overseas.

The results come after 36,362 test results were received on Friday.

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Contact tracers are working to find the local source of the highly infectious Delta strain of COVID-19 after genetic testing revealed the strain in at least two people in a family of four who travelled to regional NSW at the end of May.

The cluster now comprises seven people; the family of four who visited Jervis Bay and three new cases — two parents and a child — reported on Friday.

Health Minister Martin Foley said the two people with the new variant were unlinked cases, meaning contact tracers had not yet worked out where they contracted the virus.

One case among the remaining three revealed today is the child of the West Melbourne family that travelled to NSW and was later diagnosed with the new variant.

Another is a colleague of one of the West Melbourne family members, who was already identified as a close contact.

The third is a household contact of the first worker from the Arcare Maidstone aged care home who had been isolating during their infectious period.

Chief Health Officer Brett Sutton said it was “gratifying” to have three of five new cases in quarantine who had not been in the community while infectious.

One of the two people had been at Craigieburn Central shopping centre and came forward for testing as a result of health department efforts to test heavily in the area.

“The new cases who had not been identified as primary close contacts previously do have that link to Craigieburn Shopping Centre and Epping Plaza Shopping Centre,” Professor Sutton said.

“They were frequent attendees at both of those sites.”

That person’s partner is the other unlinked case and is a construction site worker. Their positive test result has prompted the closure of a Melbourne building site, affecting 170 workers.

Master Builders Victoria confirmed that a cleaner who was working on a ProBuild construction site at 100 Queen Street in Melbourne tested positive to the virus.

The cleaner was a subcontractor and according to the organisation solely worked at that site.

Chief Executive Officer Rebecca Casson said it was the first positive case of COVID-19 on a construction site since September 2020.

Professor Sharon Lewin from genomic sequencing centre the Doherty Institute attended Saturday’s press conference to address questions about nationwide investigations into the source of the Victorian Delta strain cluster.

She said there was no evidence pointing to the Victorian family picking up the virus while on holiday in NSW, and that her “strong hypothesis” was that it had come into the state via hotel quarantine.

“Every effort is being made right now to look for that match but we may not get the match,” she said.

She said both the Delta and Kappa strains of the virus – the latter makes up the bulk of the Victorian outbreak – are about 50 per cent more infectious than previous strains.

The Delta strain has ravaged India and is been listed as a variant of concern by the World Health Organisation.

There are also fears children are more susceptible to it than previous strains and appears to have spread further, to another family of three, through two Grade 5 children coming into contact at North Melbourne Primary School.

No-one has been hospitalised locally yet with the Delta variant, which is a “cousin” of the Kappa strain that sparked the rest of the current Melbourne outbreak.

Master Builders Victoria confirmed that a cleaner who was working on a ProBuild construction site at 100 Queen Street in Melbourne tested positive to the virus.

The cleaner was a subcontractor and according to the organisation solely worked at that site.

Chief Executive Officer Rebecca Casson said it was the first positive case of COVID-19 on a construction site since September 2020.

There are now 366 sites where exposure to the virus may have occurred.

The latest exposure sites to be added are Costco in the Docklands on May 30 between 3 and 4.30pm and 55 Collins Street – an office building in Melbourne’s CBD where a confirmed case attended for the whole day on May 25.

There are 78 active COVID cases across Victoria.

On Friday, Acting Premier James Merlino said the Commonwealth would try to help Victoria meet an increased vaccine demand.

The state wants to double the number of AstraZeneca doses available to GPs, and provide an extra 100,000 Pfizer doses from mid-June for its public vaccination sites.

-with agencies