Victorians who have been working from home must continue to do so for at least another month, as part of efforts to prevent a second wave of coronavirus.
From Monday, the obligation for employers to keep their staff working from home will be included in the Chief Health Officer's directions.
Businesses will be subject to spot checks and could face fines of up to $9913 if they force staff back to the office.
"Just to avoid any doubt we wanted to have the message as clear and as sharp as possible: if you have been working from home you must keep working from home," Premier Daniel Andrews told reporters on Friday.
The directive will be in place until at least until the end of June and is in contrast to advice in other states, where a staged return to workplaces is set to begin from June 1.
"If everyone tries to get back to office blocks and end their working from home arrangements we'll simply have too many people in close contact that will spread the virus," Mr Andrews said.
"That will cause enormous challenges and could potentially lead to a second wave. That's what we're trying to avoid."
Chief Health Officer Brett Sutton said the directive will apply to hundreds of thousands of people and will help to avoid congestion in workplaces, lower foot traffic and decrease the number of people on public transport.
"It's a really powerful additional measure that can drive down transmission of this virus," Professor Sutton said.
About 15 per cent of the state's public transport capacity could be used safely while maintaining social distancing. The current rate is about 18 per cent.
It comes as Keilor Downs Secondary College, in Melbourne's northwest, was closed for cleaning after a student tested positive to COVID-19.
"The student was probably infectious on-site on the 26th of this month and was notified yesterday," Professor Sutton said, noting the student is associated with a family cluster announced on Thursday.
He said there is no evidence of a link between the student and a staff member who also tested positive last Friday, as the teacher had no exposure to the school site.
A St Albans Secondary College student and six students at Taylors Lakes Secondary College have been identified as close contacts of the infected student, who attended a VET class while infected.
Those students, as well as 78 other known contacts, will enter a 14-day self-isolation but their schools have not closed.
Professor Sutton said a mobile testing site will be established in Keilor Downs at the Keilor Community Hub on Old Calder Highway.
"This student has acquired it from a broader family group, but how that family group acquired it is under investigation," he said.
"It might indicate that there are some low levels of transmission in or around Keilor Downs."
There were seven new cases recorded in Victoria on Friday, bringing the state's total to 1634.
Three were discovered through routine testing, two were people in hotel quarantine after returning from overseas and two are still under investigation.
One previous case was removed from the tally because of a data issue.
About 70 cases remain active.