Victoria's single new case of coronavirus is a parent of a student from a school in Melbourne's northern suburbs that was shut down earlier this week.
But Premier Daniel Andrews says there is no clear link between the new case and a current outbreak in the area.
The parent of the student from the East Preston Islamic College tested positive to the virus on Thursday.
It comes after a separate case of a Year 5 student from the college who tested positive to the virus on Wednesday, after attending school on Monday and Tuesday when they should have been self-isolating.
"Their child did not come into contact with the other positive case and that person had not been to the school. There is no link," Mr Andrews said of the new case on Friday.
"That is why it is a little bit uncertain and we have to try run that to ground. There is obviously some virus in that local community."
Some 83 families linked to the school are now self-isolating.
Asymptomatic testing will be offered to all members of the school community.
The state's Commander of Testing Jeroen Weimar said almost 19,500 tests were conducted across Victoria on Thursday, including 6500 in the northern suburbs.
Authorities had been monitoring an outbreak in the area, totalling 36 people, for a fortnight and considered it contained until the Year 5 student attended school.
The East Preston Islamic College has been closed for deep cleaning, as has the nearby Dallas Brooks Primary School after a close contact was identified there.
Eight hundred primary or secondary contacts linked to the cluster have been asked to isolate.
"It is really what we need you to do at this time to make sure we can run this virus to the ground and make sure there are no other remaining areas of community transmission," Mr Weimar said.
About 86 residents of a Broadmeadows social housing block where a positive case was detected have been cleared after testing on Thursday.
A remaining 30 residents are expected to be tested over the coming days.
"All those residents are able to travel as freely as the rest of us in line with the current restrictions," Mr Weimar said, though he urged them to continue to monitor for symptoms of the virus.
Authorities are continuing to doorknock residents of Broadmeadows, Dallas, Preston, Roxburgh Park and West Heidelberg to urge anyone with coronavirus symptoms to get tested.
Meanwhile, COVID-19 fragments have been detected in wastewater in the town of Ararat, 198km west of Melbourne, despite there being no known cases in the community
A pop-up testing site has been established at East Grampians Health Service.
"We clearly have some activity happening in the Ararat area. We are very keen to identify whether this is an old case or whether this is a new case that's developing," Mr Weimar said.
Melbourne's 14-day rolling average has fallen to 5.5.
There have been 10 mystery cases in the fortnight to October 22.
Regional Victoria's 14-day average remains at 0.3 with no mystery cases.
The state's death toll remains at 817 and the national figure 905, with only one death in the past week.
The premier said the state was "very well placed" to further ease restrictions on Sunday.
"We are confident in where things are at at the moment," Mr Andrews said.
"I can't predict what the numbers will be tomorrow but the strategy is working, the trend is with us, the numbers are falling and that is a very good thing."
He said he hoped the state would be able to facilitate international arrivals in time for Christmas.
