Melbourne remains on track for a lockdown reprieve despite authorities linking an outbreak of the more infectious Delta COVID-19 strain to a hotel quarantine case.
Acting Premier James Merlino said authorities had genomically linked the West Melbourne Delta cluster to a man in his 40s who arrived from Sri Lanka on May 8.
He assured Victorians that the latest development was not a setback to the planned easing of restrictions for Melbourne and regional Victoria at 11.59pm on Thursday.
"Today doesn't change our plans. We remain on track," Mr Merlino told reporters on Tuesday.
Emma Cassar, the head of COVID-19 Quarantine Victoria, said the infected man mistakenly opened his hotel room door early in his stay, but it would not have been long enough to transmit the virus.
"The last 24 hours have been a flurry of activity ... to make sure we cover off all bases," Ms Cassar told ABC Radio.
She added health authorities are also tracking down the people who cleaned and restocked the plane that brought the infected man to Melbourne.
Two of the 12 residents who were on the same floor of the hotel as the infected man are also being followed up, as they did not have day 17 and 20 tests.
After testing positive on the same day he returned from overseas, the man was moved from the Novotel Ibis to the Holiday Inn "health hotel" on May 9.
He completed 14 days of hotel quarantine and was released on May 23.
Genomic sequencing shows his infection is identical to one of two families linked to the North Melbourne Primary School, which has emerged as the epicentre of the West Melbourne outbreak.
However, it remains unclear how the virus was transmitted from the returned traveller - who lives in the Glen Eira area in the city's southeast - to the infected family.
Deputy Chief Health Officer Allen Cheng said there were four main theories, with the most likely being that the man transmitted the virus to a staff member while in transit or to a fellow guest.
That person has then gone on to infect someone in the community.
"I am very surprised it got out. But this is what we have, and we need to work out what has happened here," Professor Cheng said.
Acting Police and Emergency Services Minister Danny Pearson said no one who came into contact with the man has tested positive so far, including fellow plane passengers and crew, Skybus and hotel quarantine staff and other hotel guests at the Novotel Ibis.
Meanwhile, none of the guests at the Holiday Inn at the time of the man's stay were infected with the Delta variant.
Mr Pearson said there was no indication that transmission had taken place inside either of the hotels as COVID-19 Quarantine Victoria continues to review CCTV footage and incident logs.
Victoria's deadly second wave of COVID-19 last year was sparked from leaks in hotel quarantine, leading to a major overhaul of the system.
The revamped program restarted in December, only for it to be paused in February as a leak from the Holiday Inn grew to nearly 25 cases and triggered a three-day lockdown.
Last week, Victoria secured federal government support to construct a purpose-built quarantine facility in the state.
Victoria reported two new local COVID-19 cases on Tuesday, both linked to existing outbreaks and with limited community exposure.
One is a child linked to the West Melbourne outbreak, while the other is a household contact of a worker linked to the Arcare Maidstone cluster.
The state's COVID-19 commander Jeroen Weimar said a string of new exposure sites added to the list of more than 270 on Monday evening were linked to the new cases.
Some 22,814 Victorians were tested on Monday and 21,192 received a vaccine dose at state-run sites.