Speculation is mounting that Melbourne will be forced back into lockdown, after the latest cluster increased by 11 cases overnight.

The state's health authorities spent the night debating whether to impose a lockdown of up to seven days. It would be the fourth time Melbourne has been placed in a lockdown.

Victoria's Covid-19 outbreak has risen significantly overnight, with the state's cluster now at 26 cases.

One of the 12 cases confirmed by the Department of Health was reported on Tuesday, meaning there were 11 new infections recorded overnight.

There has been ongoing speculation about whether parts of the state will be plunged into a lockdown today, with some reports suggesting a five or even 10-day lockdown could be on the cards.

Government officials met late into Wednesday night to debate the state's next move, with reports further meetings are being held this morning to nail down details of the lockdown.

There are now 79 "exposure sites" visited by the cases - including a McDonald's, Anytime Fitness gym and a Kmart. are among a long list of new venues added to a list of exposure sites, which has now risen to 79.

Victorians woke up on Thursday anxious to learn if an ongoing coronavirus outbreak that began in Melbourne last week would lead to new snap lockdown measures.

Epidemiologists have suggested that an increase in cases in the current 24-hour timeframe could be the trigger needed for authorities to lock down Melbourne and impose further restrictions further afield.

As contact tracers do their best to trace everybody who has come into contact with the confirmed cases, exactly how the outbreak began is still a mystery.

What do we know so far?

Genomic sequencing has confirmed the cases that are popping up around Melbourne all stem from South Australia's hotel quarantine system.

Earlier this month, Victoria's health authorities put out location alerts out for six potential Covid-19 exposure sites.

This was triggered by the movements of a man who completed his hotel quarantine in South Australia returned to Victoria and tested positive. It became known as the Wollert case, after the suburb on Melbourne's northern fringes.

Three of the man's household contacts returned negative test results, there was no lockdown and life resumed as it was before.

Commuters wear masks on the tram in Melbourne this week. (Photo / Getty)

Seemingly a bullet had been dodged, but now it's clear that something has gone wrong.

Having linked the current outbreak back to this single traveller, authorities say there is still a "missing link".

Health authorities in Victoria are trying to find out how the Wollert case is connected to the current outbreak, but it is proving difficult given the traveller may have infected someone who did not develop symptoms and, therefore, didn't get tested.

Cases suddenly appear

The first case to be discovered in Melbourne in the current outbreak was a man in his 30s, who got tested on Sunday, but that certainly doesn't mean he was the first to be infected.

As far as we know so far, the fifth case to be picked up, a man in his 60s, was the first to become infectious.

He first displayed symptoms of coronavirus on May 17 and got tested on May 24.

He met up with the man in his 30s for a business meeting on May 18 — which is how health officials believe the virus was passed on.

The man in his 30s unknowingly passed the virus on to four members of his household, including a preschool child, a grandfather in his 70s and a woman in her 70s.

Those were the four cases that were announced on Monday. With the man he met being reported on Tuesday morning that brought the total number of cases in that 24-hour period to five and triggered new restrictions.

The man in his 70s is the case who visited Highpoint Shopping Centre on May 20 which led to a number of exposure sites there.

Cases start to spread

It has become clear in the past 24 hours that the outbreak is spreading further afield — with 10 new infections confirmed.

The outbreak is concerning authorities because one of those new family cases went to work at a finance firm, Stratton Finance, in Port Melbourne — where five further people have been infected in the latest figures.

It shows that the outbreak is moving beyond Melbourne's northern suburbs where it was first detected, and officials are frantically chasing down contacts of those who work at the finance firm.

A nurse conducts a Covid-19 test at a drive-through testing facility at the Melbourne Showgrounds on Tuesday. (Photo / Getty)

Stratton Finance executive director Toby Simmons told the AFR the company has up to 100 people in the head office but there were far fewer people attending this week.

He said all staff who attended the office have been notified, tested and are self-isolating. The company, located on Williamstown Road, specialises in car, boat and caravan finance.

Another case was picked up in the past 24 hours — a contact of the man in his 30s who was the first case to be picked up.

Critical 24 hour phase

Speaking at a press conference yesterday, Acting Premier James Merlino revealed he was "very concerned" about the spike in cases, prompting further fears Melbourne could be sent into a snap lockdown.

A raft of new restrictions were introduced across Greater Melbourne on Tuesday, but Merlino warned the government hadn't ruled out taking further action.

"I want to be upfront with everyone this morning – I cannot rule out taking some further action, but we'll update people as soon as we know," he said.

"It's good that everyone is linked at the stage, but we are concerned about the number, and also about the kind of exposure sites, and the next 24 hours are going to be pretty critical if we're going to make any further changes beyond the changes that we announced yesterday."

It is possible tougher restrictions could even be brought in by the end of the day, Melbourne University epidemiologist Professor Tony Blakey told radio station 3AW.

He said the situation has, in a short period of time, "really escalated to become quite concerning" and things were now "really serious".

"I'm expecting that we will see significant increases in restrictions announced soon. We do unfortunately have a problem on our hands," he told the radio show.

Professor Blakey said if a lockdown does occur, it likely won't be as short as some of the other lockdowns other parts of Australia have experienced in recent months.

"If we go into lockdown, I don't think this will be circuit-breaker. It would be for at least five days and be reassessed as the numbers come through. It wouldn't just be a three-day one, I suspect," he said.

The current restrictions, which were introduced at 6pm Tuesday, include:

• A limit of five visitors per day for private households
• A limit of 30 people for public gatherings
• Masks required indoors for everyone aged 12 and over — even in workplaces — unless an exemption applies.

So far 301 primary close contacts of confirmed coronavirus cases have been identified, with 80 of those returning negative test results.