Authorities have held late-night crisis talks over the worsening UK mutant strain outbreak in Melbourne, planning for a possible lockdown in days.

There are fears that Melbourne is on the brink of its third lockdown this morning as a cluster of cases linked to Victoria's hotel quarantine system grows to 13.

It is understood that state government advisers overnight met to draw up a framework for another lockdown, which could be introduced as early as Friday night, or possibly within days.

Authorities are not only concerned about the growing Holiday Inn cluster, but they are also understood to have worried about virus fragments detected in wastewater across Melbourne.

A source close to Emergency Management Victoria told the Herald Sun authorities feared they had lost control of the outbreak — describing scenes of "pandemonium" at the agency.

They told the newspaper there were deep concerns at the failure of contact tracers to match information they had been given by confirmed cases and their close contacts with what the results of sewage testing was showing about the virus' spread.

Officials are working with the theory that all of the cases linked to the Holiday Inn outbreak are UK strain cases — meaning it could spread a lot more quickly than the strain that took hold of Victoria last year.

In response, the Herald Sun reports Victorian health authorities are weighing up a snap lockdown, much like the five-day shutdown in Perth at the beginning of the month.

In Western Australia, residents were only allowed to leave home for an hour to exercise with a mask within 5km, or if they were an essential worker, needed groceries or medical supplies, were receiving healthcare or were supporting someone with needs.

Schools, gyms and cinemas were also shut, while restaurants and cafes could only serve takeaway.

Under those rules, crowds would also be banned from the Australian Open.

Clinical epidemiologist, Professor Nancy Baxter, appeared to back the idea of another lockdown in the city this morning — after two more people linked to the Holiday Inn coronavirus outbreak in Melbourne tested positive for coronavirus overnight and more exposure sites were listed.

"What I am concerned about is as the sites of the at-risk sites grow and grow, there is the potential there has been some kind of spread outside of that group of close contacts that is already in quarantine," she told Today this morning.

"It could be a week or more before we know it. And so then there is that opportunity for spread throughout the community, under our very noses.

"I think if there is a time to really get this under control the time probably is now. So it is a very tough decision and, you know, if there is not any further community spread it always looks like this was out of proportion, but I must say you can't prove prevention.

"So I think this is something that the Victorian government needs to be thinking about very hard today in terms of whether the best thing for us is to go into lockdown so we don't have a third wave in Victoria."

Both of the latest Covid-19 cases linked to the Holiday Inn cluster are household primary close contacts of previously announced infections.

It brings the total outbreak to 13, with six of the cases already confirmed by genomic sequencing as having the UK's highly contagious B117 strain.

Victoria Health also added Brunetti at Melbourne Airport's Terminal 4 to the list of Tier 1 exposure sites overnight. Anyone who visited the cafe between 4.45am and 1.15pm on Tuesday February 9 must get tested and remain isolated for 14 days.

Earlier, a staff member at the Holiday Inn in Tullamarine became the latest infection linked to the cluster. Victoria's deputy secretary of community engagement and testing Commander Jeroen Weimar said at that point that it was a "working assumption" that all cases associated with the hotel cluster were of the UK variant.

"Clearly it is a very live outbreak, we are at this stage reassured by the fact that all of these positives emerged from a primary contact field, that is important to us.

"And although we are now seeing two cases of household transmission, again it's in the household, that gives us some confidence, but that is early days."

It comes as Australia's chief medical officer Professor Paul Kelly said he will investigate claims that a man breached hotel quarantine requirements in the state and slipped through hotel staff to deliver a PlayStation 4 to a friend in quarantine.

The potential breach at the Park Royal hotel is the latest blow for Victoria as it grapples with its growing cluster near Melbourne airport.

At a press conference on Thursday, Kelly said he would "follow it up" and that the situation was "not ideal".

"What you describe there is not ideal of course, we want quarantine to be exactly that, to separate people from the wider community to minimise the chances of the virus spreading.

"So that's not an ideal situation and I'm sure the Victorian authorities are looking into that, but I'll follow it up."