A Bunnings and a Coles supermarket have been added to Melbourne’s coronavirus exposure sites as authorities try to trace the source of a mystery cluster.

The four infections confirmed in Victoria on Thursday came just hours before Melbourne’s extended lockdown was due to end.

Victorian authorities say the source of the infections in the family, from the northern Melbourne suburb of Reservoir, remains a mystery.

None of them are primary close contacts of anyone in Melbourne who has had COVID recently, or the hundreds who remain in isolation as the city’s outbreaks are closed down. Nor have they visited any of Victoria’s 180-plus exposure sites.

“We continue to investigate. The focus for us today and over the last 12 hours was to establish the link to the Reservoir unknown cluster, and how it connects. There are a number of leads that we are working through,” Victorian testing commander Jeroen Weimar said on Thursday.

A man in his 80s was the first family member to test positive on Wednesday.

The other ill family members are a woman in her 70s and two men in their 50s and 20s. None are in hospital.

The man in his 80s developed symptoms on Monday and was tested on Tuesday. The short window means only four initial tier two exposure sites have been linked to the family unit.

Victorian deputy chief health officer Allen Cheng said the investigation into the family’s cases was in its early stages.

“We would be more comfortable if we could identify a link … to another cluster,” he said.

New exposure sites from the latest cases include:

• Marco Fine Food and Groceries, Reservoir
• BP service station, Thomastown
• Bunnings, Thomastown
• Coles, Bundoora Square
• Others are likely to be added – and can be found here

Masks will remain mandatory when Melburnians leave home, even after the lockdown lifts.

Despite the mystery cases, Melbourne and regional Victoria will still move to eased restrictions at midnight on Thursday – with one small exception.

Acting Premier James Merlino confirmed rules would be wound back as planned, although masks will remain mandatory indoors and out in Melbourne.

“It is inconvenient but an easy thing to do,” Mr Merlino said ahead of the city’s lockdown lifting.

“We are used to it, we know it works.”

Victorian authorities have also flagged the likelihood of the post-lockdown restrictions remaining for longer than a week, depending on what their investigations of the Reservoir cluster uncovered.

Also on Thursday, they were expecting to interview a Melbourne couple who have prompted COVID scares in Queensland and NSW.

They left Melbourne on June 1, when the city was in lockdown, and tested positive at the end of a road trip through NSW and into Queensland.

Both have since tested positive to the virus, although it appears they are late in their infection period.

“It is important to note that if they were relocating it is not a breach of directions here in Victoria. But we just don’t know,” Mr Merlino said.

While unable to initially pinpoint a possible source of the couple’s infections, Professor Cheng said one of them had checked-in near the Craigieburn Central shopping centre on May 23.

There have been nine cases linked to the shopping centre in the city’s north.

“The fact that we’ve been able to identify a possible link to the Craigieburn shopping centre within hours of hearing about these cases and even before being able to speak to these cases, really highlights the value that we have in QR codes,” Professor Cheng said.

Mr Weimar said 1500 primary close contacts had been cleared in the past 24 hours.

There are 78 active COVID cases across Victoria, down five from Wednesday.

In other developments on Thursday, NSW will lift stay-at-home orders for anyone who has been in Victoria since May 27 from 12.01am on Friday, while New Zealand has extended the pause on quarantine-free travel from Victoria for at least another week.

The federal government’s hotspot declaration for Melbourne will end at the same time as the lockdown does, at 11.59pm on Thursday.

-with AAP