The state opposition criticised the government for not committing to fine attendees, which the opposition said would have dissuaded some from attending.

But Professor Sutton defended the public messaging from health authorities. "I don't know that I could have said it any more strongly than that ... I was entirely clear about the potential risk of transmission."

He said the fining of organisers was a matter for the police, who on Wednesday afternoon said they were still investigating and "expect to take enforcement action against several organisers in due course".

"I'm not an expert in how you manage protests," Professor Sutton said.

"Clearly protests can happen regardless of the legal consequences. They're difficult to manage."

Professor Sutton said a "significant" investigation would take place to determine the source of the man’s infection and who he had contact with at the protest. He said he was not sure whether the man had the government's COVIDSafe tracing app on his mobile phone.

While the true spread of the virus among protesters will become clear 10 to 14 days after the June 6 rally, Victoria's health chief said it was possible this case could have sparked a chain of infection.

"Every individual will be different … 70 to 80 per cent of people don't transmit to anyone else. But there are a smaller proportion, 20 to 30 per cent of people, who will spread it to a number of other individuals and close contacts," he said.

Professor Sutton added that being outdoors and wearing a mask would likely mitigate the spread of COVID-19 "but it doesn't completely protect those around you".

"People were trying to distance in that gathering but it's effectively impossible to [keep] the 1.5 metre requirement," he said.

Prime Minister Scott Morrison has said he wants anyone who attends future rallies arrested and charged.

Mr Morrison said the "double standard" displayed by protesters had offended many Australians and was a "real blocker" to easing coronavirus restrictions.

"It just puts a massive spanner in the works and that's why it's so frustrating," Mr Morrison told 2GB radio on Thursday.

"They have put the whole track back to recovery at risk and certainly any further action on this front would be absolutely unacceptable."

The AFL season resumes on Thursday evening in Melbourne and Professor Sutton said his advice on allowing fans into matches - like South Australia, Queensland and NSW will this weekend - would centre on achieving a "feasible crowd size".

"[It may be] a case of deferring the opening, but opening at a point where we can be very confident that 10,000 people can gather together, and that's not out of the question in the next couple of months, but we have to see how numbers are going," he said.

A toddler at the Professor Lyn Corcoran Early Learning Centre in Parkville also tested positive.

The child attended two days of childcare while potentially infectious but asymptomatic. The centre, which is attached to the Royal Children's Hospital and is largely attended by the children of staff, will close for at least 24 hours as it is professionally cleaned. Children and staff at the centre will be tested.

A further case was detected in a former staff member at BUPA Aged Care in Clayton, although the person finished their employment at the facility on June 4 and did not work there while infectious.

Another new case was a close contact of a Rydges on Swanston staff member, taking the size of the outbreak linked to the hotel to 15. Two other cases were returned travellers in hotel quarantine.

Roads Minister Jaala Pulford also announced on Thursday that drivers' licence testing will resume on Monday, although learners will have to wait around three months to get a new appointment to sit their test.

A backlog of 55,000 driving tests which were cancelled when testing was suspended on March 25 will be prioritised, with six pop-up testing sites to be created and 200 new staff to be brought in.

with AAP