A man who was infected with the Delta COVID-19 strain mistakenly opened his room door soon after entering hotel quarantine in Melbourne.
But Emma Cassar, the head of COVID-19 Quarantine Victoria, says this incident is unlikely to be the reason for the Delta variant spreading.
Health authorities have linked an outbreak of the more infectious strain in Melbourne to the hotel quarantine case, a man in his 40s who returned from Sri Lanka on May 8.
They are now scrambling to work out the connection.
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.
But it remains unclear how the virus was transmitted from the returned traveller - who lives in the suburban Glen Eira area in Melbourne's southeast - to the infected family.
"The last 24 hours have been a flurry of activity ... to make sure we cover off all bases," Ms Cassar told ABC radio.
The man initially stayed at the Novotel Ibis Hotel before testing positive and transferring within 24 hours to the Holiday Inn "health hotel".
During his stay at the Ibis, the man opened his door while a staff member was in the corridor.
"The only IPC (infection prevention control) breach is when this resident opened (his) door for less than a minute - he thought there was a knock on the door (but) it was a room further up," Ms Cassar said.
"He even acknowledged the staff member on the floor was more than six metres, so it's not enough for a transmission event.
"He would have had very low levels of infectiousness at that point."
She added the worker in the corridor was wearing an N95 mask and a face shield.
The City of Whittlesea cluster, which triggered Melbourne's current lockdown and is separate to the Delta cluster, is linked to a Wollert man who contracted the virus while in Adelaide hotel quarantine.
A report said the Wollert man probably caught the virus because of the opening and closing of doors in the hotel corridor.
Ms Cassar said two fellow hotel quarantine residents of the man infected with the Delta strain were also being followed up for testing.
She revealed the two residents did not have day 17 and day 20 tests, adding these were not mandatory.
Asked why there weren't, she told ABC radio that was a question for public health.
"They are being followed up now, just to run that one to ground," she said of the two residents.
All 12 residents on the same floor are also being asked to submit to further testing.
Ms Cassar said health authorities were also tracking down the people who cleaned and restocked the plane that brought the infected man to Melbourne.
She said 265 of 268 hotel staff at the Ibis had tested negative.
Deputy Chief Health Officer Allen Cheng said there were four main theories for how the Delta strain spread, with the most likely being the man transmitted the virus to a staff member while in transit or to a fellow guest.
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.
Meanwhile, acting Premier James Merlino says Melbourne and regional Victoria remain "on track" for eased restrictions at 11.59pm on Thursday.
Victoria had two new cases on Tuesday and they are linked to existing infections.
© AAP 2021