Victoria has confirmed another two locally-acquired coronavirus cases, taking its total to eight.
The increase has prompted a hard border with NSW from 11.59pm on January 1, acting premier Jacinta Allan said on Thursday afternoon.
Anyone who arrives back in Victoria from anywhere in NSW after that time will have to go into isolation for 14 days.
"This is going to cause some disruption for Victorians who may be holidaying," Ms Allan said, apologising for the disruption.
"However these difficult decisions are about protecting the community, protecting and keeping case numbers low and doing everything we can to lock in the gains we have made over the course of 2020."
Masks will be mandatory indoors from 5pm on Thursday and the number of visitors allowed in Victorian homes is down to 15 from 30 under changes announced by Ms Allan earlier in the day.
The two additional cases identified on Thursday afternoon are close contacts of the six previously announced.
The new cases, which first emerged on Wednesday night, ended a 60-day streak without infections for Victoria.
It's believed all six cases - including two women in their 40s and a woman in her 70s - are tied to outbreaks of the virus in Sydney.
All are directly or indirectly linked to the Smile Buffalo Thai restaurant in Black Rock on December 21, which was attended by a NSW returned traveller.
That traveller returned before border permits were in place and is being tested on Thursday.
Victoria's testing chief Jeroen Weimar said early information indicated a potential exposure window between December 17 and 19.
Earlier on Thursday Health Minister Martin Foley has urged Victorians not to travel to NSW and for any Victorians in the previous NSW green zone to return immediately.
"You do not want to be caught on the wrong side of a rapidly evolving situation," he said.
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.
Anyone already in Victoria who travelled to those regions since December 27 should get tested and stay at home until their results come back.
Opposition Leader Michael O'Brien said Victorians shouldn't panic and new cases were inevitable.
"What's really important is we keep in place the measures to keep us safe," he said, supporting the latest restrictions.
Celebrations for the Victorian new year 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 57-year-old NSW woman was arrested after speeding through a checkpoint in East Gippsland on Wednesday, coming from a designated red zone.
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