Victorian authorities have had a breakthrough in tracing the origins of a worrying outbreak of the Delta variant of the coronavirus that has so far infected 15 people.

Acting Premier James Merlino said on Tuesday genomic testing had linked the outbreak to a traveller who went into hotel quarantine in Melbourne after arriving from Sri Lanka on May 8.

He tested positive on his first day in Australia.

Authorities are still trying to find a link between the traveller and the infected families, who centre around a North Melbourne school.

The West Melbourne Delta outbreak grew by another case on Tuesday, one of just two new virus infections reported in Victoria. The other is a household contact of a worker at Arcare Maidstone, the aged-care home in Melbourne’s north-west where 10 COVID cases have now been linked.

Despite the small daily increase – which took Victoria’s outbreaks to 83 cases – Mr Merlino dashed any hopes of an early end to Melbourne’s extended lockdown.

Stay-at-home orders for millions of Melburnians are due to ease at midnight on Thursday.

“Nothing about today changes our plan,” Mr Merlino said.

“We remain on track to later in this week announce, as we have said we have planned to do all along, further easings of restrictions in regional Victoria and careful easing of restrictions in Melbourne.”

He has previously flagged that Melbourne is likely to move to similar restrictions as in regional Victoria since the statewide lockdown eased, with people unable to travel during the Queen’s Birthday long weekend.

Deputy chief health officer Allen Cheng said the genomic breakthrough in the Delta spread gave contact tracers a source and a date for the first infections in the cluster.

“We now know it couldn’t have been in Victoria before May 8 and that is significant because it means there is unlikely to be very widespread transmission for a long period of time before this outbreak was detected in North Melbourne,” he said.

Authorities are also reconsidering their theory that the West Melbourne family who fell ill after a holiday in Jervis Bay, on the NSW south coast, were the first infected.

The Sri Lankan returnee’s virus is a perfect genomic match for the second family identified in the West Melbourne outbreak.

“We are investigating the possibility that the second family was the first to get infected and then have transmitted it to the other family,” Professor Cheng said.

He confirmed a likely easing of the lockdown later in the week.

“There are also other things that would give us that reassurance – that there’s been enormous turn out for testing. I think it’s close to 10 per cent of Victoria has been tested in the past couple of weeks,” he said.

“It would be hard to think there is undetected transmission with that level of testing at least.”

The Delta variant, which originated in India, is considered up to 40 per cent more infectious than earlier coronavirus variants. Last week, when the cases emerged in Melbourne, health authorities said it had not been seen before in Australia.

Mr Merlino also took another swipe at hotel quarantine, following confirmation of the outbreak emerging from an infected Melbourne returnee.

“We all know that hotel quarantine cannot be made risk-free. Hotels are built for tourists, not for managing infectious diseases,” he said.

“There have been at least 21 breaches in hotel quarantine right around this country. Just in the last month, we have seen three outbreaks, one in South Australia and two in WA.”

Tuesday’s COVID update came as the number of exposure sites across Victoria remained high.

A construction site in Melbourne’s CBD is one of the recent exposure sites. Photo: Getty

Many were removed on Monday, after two weeks since their first exposure. But more than a dozen more were added shortly after, taking the number back up above 300.

A busy Melbourne CBD construction site is a key remaining focus for contact tracers. Authorities have confirmed the virus spread from a cleaner who had tested positive to another worker at the ProBuild site at 100 Queen Street about the start of June.

There are 492 close contacts linked to the site, with more than 400 returning negative tests by Monday night.

Other CBD exposure sites include Woolworths Metro on Little Collins Street, Aki Sushi on Queen Street, and two buildings on Collins Street.

More than 5500 people are still in quarantine, but about nearly 1500 have finished quarantine this week.

More than 550,000 tests have been administered in Victoria since the outbreak started two weeks ago.