The non-indigenous man in his 30s wore a mask, was not symptomatic at Saturday's protest and is unlikely to have contracted the virus at the event, Chief Health Officer Brett Sutton told reporters.
But he did develop symptoms within 24 hours of attending.
"They were potentially infectious, so the lesson about warning people not to attend applies. The directions in place which limit mass gatherings ... are there for a reason," Prof Sutton said on Thursday.
The man is one of eight new cases recorded in Victoria overnight.
Two cases are travellers in hotel quarantine, one is a close contact linked to the Rydges hotel cluster and another worked at Bupa Aged Care in Clayton. A toddler at childcare centre in Parkville has tested positive and two further cases are under investigation.
The Rydges cluster now stands at 15.
The Bupa aged care staff member was not infectious while working at the facility, according to Prof Sutton.
Authorities are not sure how the toddler acquired the virus.
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How can NZ open it's border to Australia when they clearly still have community transmission of Covid-19?
Couple that with failures to follow government protocols for people in isolation within NZ and you have a very real possibility of a second wave of the disease.
Some would say it is going to happen anyway, given the poor state of global cases, but that doesn't mean we should just blindly accept it.
Who wore a mask and was not symptomatic at saturdays protest and is unlikely to have contracted the virus at the event. But he did develop symtoms within 24 hrs of attending. So this is just a coincidence ???? Maybe a trans tasman bubble is further away than we hoped