One of today's five cases is a "mystery case" – a female worker at a Melbourne aged care facility.
Testing commander Jeroen Weimar said the case was "extremely concerning".
The woman worked at Arcare in Maidstone on Wednesday and Thursday.
READ MORE: Victorian government announces $250 million support package to struggling businesses
The facility is now under an additional lockdown with residents confined to their rooms.
Mr Weimar said the employee became symptomatic on Friday and had received a first dose of a COVID-19 vaccine.
"I can confirm that this facility has been on the Commonwealth program for vaccination, over half the residents have been vaccinated, and we believe about one-third of the staff have been vaccinated," he said.
READ MORE: Queensland tipped to convert sports stadiums into mass vaccination hubs
"This individual positive case member has had her first vaccination dose.
"Again, we got strong records with this particular facility and my thanks to them, but I would please encourage anybody who has got any connections with ArcCare in Maidstone on Thursday the 26th of May and Friday the 27th of May - to please getin touch immediately, isolate and get tested."
"I want to thank the worker for doing the right thing and getting tested immediately upon getting symptoms, and helping get the facility as safe as possible," he said.
In total, two of today's five cases are close contacts in quarantine, one is linked to an exposure site and two are under investigation.
