Authorities are still discussing how many days it will last. It is not yet known if the lockdown will also apply to regional Victoria.
It will be the fifth lockdown since the pandemic began and the third this year.
Mask rules were already tightened overnight to respond to the outbreak, which is now linked to 16 cases.
Two of those were confirmed this morning, adding to the 10 included in yesterday's numbers.
The new cases are understood to have acquired the virus at an AFL game at the MCG on Saturday which was also attended by a COVID-positive case.