COVID-19 Victoria outbreak: How did the northern Melbourne cluster begin?

Victoria will enter a seven-day 'circuit-breaker' lockdown at 11:59pm on Thursday (local time) as the state's spiralling outbreak of COVID-19 continues to escalate - but how did it get to this point?

On Thursday morning, Victoria Department of Health confirmed an additional 12 people tested positive for the virus in the 24 hours up until midnight. One of these cases was first reported on Wednesday, but wasn't included in the official figures. The remaining 11 are new. 

The latest infections are all linked to the existing outbreak centred in Whittlesea, a local government area in Melbourne's outer northern suburbs. The cluster now stands at 26 cases. 

The situation has escalated rapidly for Victoria, a state that had enjoyed an 86-day streak without locally acquired cases - or community transmission - prior to the outbreak.

Although the first batch of cases did not start emerging until Monday, the outbreak has been linked back to a man who tested positive more than two weeks ago. 

Case zero: Wollert man returns from quarantine in Adelaide, SA

This man had returned to Australia from India on April 19. He completed his two weeks of quarantine at the Playford Hotel in Adelaide, South Australia, and tested negative on days one, five, nine and 13 of his 14-day stay.

He was discharged on May 4 and returned to his home in Wollert, a northern suburb of Melbourne. He developed symptoms on May 8 and tested positive for the virus on May 11. The man's three household contacts and close contacts at work were all tested and returned negative results.

Before the man tested positive, he went to work for several days and attended a Woolworths supermarket in Melbourne's north while infectious.

The Health Department wrongly listed Woolworths Epping as an exposure site. The department later confirmed it was Woolworths Epping North, about four kilometres away, but it took contact tracers almost two weeks to realise the mistake.

The man was placed back into quarantine at a hotel facility in Melbourne. It was later confirmed he had become infected while quarantining in Adelaide. It's understood he contracted the virus from another guest due to airborne transmission. 

Speaking at a press conference on Thursday, health officials told reporters the transmission likely occurred through the hotel corridor due to open doors.

Traces of virus discovered in Wollert wastewater samples

On Friday, May 21 - 10 days after the man returned a positive result - traces of the virus were detected in samples taken from wastewater surrounding Wollert and Epping, another northern suburb.

As aforementioned, the man had returned to his home in Wollert prior to testing positive. 

The sewage fragments set off alarm bells for Victorian health authorities, as at that point, the man had been back in quarantine for 10 days.

That day, Victoria's Chief Health Officer Brett Sutton issued an alert to Epping residents, warning locals to be "aware of" the emerging situation. 

Acting Premier James Merlino.
Acting Premier James Merlino. Photo credit: Getty Images

Two 'likely cases' announced, four people later test positive on Monday

On Monday morning, the Victoria Department of Health announced that two people "likely" had the virus, also known as probable cases. Authorities said the two were linked and would undergo urgent retesting. 

Later that day, it was confirmed both individuals - male relatives in their 30s and 70s - had tested positive for COVID-19, as well as two others. The two additional cases - a woman and a preschool-aged child - were also relatives, but from separate households.

Five people test positive on Tuesday

On Tuesday, it was reported that a fifth person, a man in his 60s, had contracted the virus. He was reported to the department in the early hours of Tuesday and was formally entered into the system after midnight. He had been identified as a contact of the first case, the man in his 30s, and was urgently tested, which returned the positive result.

Later, it was confirmed that four of the man's household contacts had also tested positive, bringing the cluster to nine. 

Chief Health Officer Brett Sutton said just 80 of a potential 300 primary contacts had received their results back, with the rest to be returned on Wednesday. 

Authorities now believe the fifth man, who was symptomatic and out in the community for days before being tested, could have infected many of the others in the cluster. He is considered the origin case in the outbreak, as he reported becoming symptomatic earlier than the first case who tested positive on Monday.

The good news came when genomic sequencing was able to link the new infections to patient zero, the Adelaide quarantine case - however, early investigations found the cases had no connection to the associated exposure sites.

It was established that there was a missing link between the Wollert man and the latest bout of infections. 

That afternoon, New Zealand's COVID-19 Response Minister Chris Hipkins confirmed quarantine-free travel between Victoria and New Zealand would be paused from 7:59pm (NZT) for 72 hours.

Six people test positive on Wednesday 

Health officials confirmed on Wednesday that six additional people had tested positive for COVID-19, each with strong links to other cases within the cluster.

The outbreak then stood at 15, all of whom were traced back to the man from Wollert who contracted the virus in an Adelaide quarantine hotel.

Cases 10, 11, 12, 14 and 15 are workplace contacts of case nine, a family member and household contact of the fifth case. Case 13 is a close contact of case one, the man in his 30s who tested positive on Monday and was first reported as a "likely" case.

Eleven people test positive on Thursday

On Thursday morning, Victoria Department of Health confirmed there were 12 new cases of the virus - 11 of which were new, with one from the previous day added to the official figures. 

The 11 cases are all connected to the existing outbreak, health officials confirmed.

However, there are in excess of 10,000 primary and secondary contacts who will need to either go into quarantine or test and isolate - and that number will continue to grow, Acting Premier James Merlino said during Thursday's press conference. 

"In the last day, we've seen more evidence we're dealing with a highly infectious strain of the virus, a variant of concern, which is running faster than we have ever recorded," he said.

Merlino said contact tracers have locked down three rings of contacts within 24 hours - "the fastest our contact tracers have ever moved within a 24-hour period".

"Despite working as fast as this, and my thanks to all of our contact tracers, this variant is moving faster still," he continued. 

"The time between catching the virus and passing it on is tighter than ever. For some of those cases, how long it takes between the onset of symptoms in the first and secondary case is averaging just over a day.

"Now, to put that in some perspective, the usual transmission is about five to six days."

He subsequently announced that the state of Victoria would enter a seven-day "circuit-breaker" lockdown at 11:59pm on Thursday (local time). During this time, residents are only able to leave their homes for five reasons: to shop for food and essential items; for caregiving, compassionate, or medical reasons; for exercise, with one other person, for a maximum of two hours within a 5km radius of the home; to attend authorised work or permitted education; and to get vaccinated.

Masks are now mandatory in all indoor and outdoor settings - other than the home - for those aged 12 and over.

And as Victorians prepare for a week of lockdown restrictions, the missing link has yet to be solved, with health officials still unsure how the transmission occurred.