The new cases are all close contacts of two women in their 40s and a woman in her 70s from Mitcham, Hallam and Mentone who were revealed on Wednesday night as having tested positive.
It's believed all the cases are tied to outbreaks of the virus in Sydney.
The Mentone and Hallam women are from one family and were at the Smile Buffalo Thai restaurant in Black Rock at the same time as a close contact of the Mitcham woman and a New South Wales returned traveller, it was revealed on Thursday.
That traveller returned before border permits were in place and is being tested on Thursday morning.
Victoria's testing chief Jeroen Weimar said early information indicated a potential exposure window between December 17 and 19.
Contact tracing has identified more than 50 primary close contacts of the women, who are isolating at home including in Leongatha and Barwon Heads.
Acting Premier Jacinta Allan has imposed new restrictions for Victorians celebrating the new year, with a new limit of 15 visitors to households and reinstated mandatory masks at all indoor venues.
Health Minister Martin Foley has urged Victorians not to travel to NSW and for any Victorians in the NSW green zone to return immediately.
"You do not want to be caught on the wrong side of a rapidly evolving situation," he said.
"Now that we have links to the NSW outbreaks in Victoria, we are having to respond really quickly to get on top of that and a part of that is to make sure that as a situation seemingly continues to deteriorate in NSW, that we respond appropriately."
The new Black Rock exposure site is on top of a series of locations released by the health department late on Wednesday, with exposure dates between Boxing Day and December 28.
They include Katlialo restaurant in Eaton Mall in Oakleigh; Mentone/Parkdale Beach; Century City Walk and Mocha Jo's in Glen Waverley; Kmart, Big W Target, Millers, King of Gifts and Lo Costa stores at the Fountain Gate Shopping Centre, and the Holy Family Catholic church at Doveton.
Victoria has also announced tighter border restrictions with NSW, adding to bans on the northern beaches, Greater Sydney and Central Coast.
From Friday, anyone travelling from Wollongong and the Blue Mountains in NSW will not be allowed to enter Victoria.
Anyone already in Victoria who travelled to those regions since December 27 should get tested and stay at home until their results come back.
Until Wednesday, Victoria had been on a 60-day streak of no locally acquired coronavirus cases.
Despite the run, celebrations for the new year in Victoria were already going to be muted on Thursday night.
"It has been a very hard year and we have done an incredible thing as Victorians - let's not risk it as we celebrate the end of 2020," Police Minister Lisa Neville said.
Meanwhile, a NSW woman has been arrested after speeding through a checkpoint in East Gippsland on Wednesday.
The 57-year-old is from Bateau Bay on the NSW Central Coast, a designated red zone.
Police had to use a tyre deflation device to stop her car.
