Premier Daniel Andrews says he plans to announce the length of the extension on Tuesday morning.

There were 13 new local infections in the 24 hours to Monday morning, one of which was a case in Mildura, previously announced by authorities.

Mr Andrews said there were too many chains of transmission for the lockdown to lift at the original time of midnight Tuesday.

"We are running alongside this virus, but we're not yet in front of it," he told reporters on Monday.

"We've gone hard and early and I think we have avoided, in fact I know we have avoided something altogether worse."

Mr Andrews said testing figures, exposure sites and test results on Monday would guide the extension duration.

All the new cases are linked to the current outbreaks of the Delta variant in the state, which originated in NSW.

Among the cases is a household contact of the Mildura case revealed on Sunday, two are Trinity Grammar students, another is a close contact of a Phillip Island case, one is a case linked to AAMI Park and another was at the Euro 2020 viewing at Crafty Squire pub in the Melbourne CBD.

"That final case that Crafty Squire is of concern. It is absolutely an example of how quickly this variant is moving in the short time we are seeing between exposure and then being infectious and onward transmission," Chief Health Officer Brett Sutton told reporters.

"So while we acted as conservatively as possible in declaring Crafty Squire a high-risk venue, we didn't necessarily expect there to be transmission there. The fact it has occurred some 30 hours after being exposed is absolutely a feature of the Delta variant."

With more cases appearing connected to the Melbourne Cricket Ground and AAMI Park cases, contact definitions have been expanded and authorities are asking for more people to go and get tested.

Another four cases have also been recorded on Monday, that will be put on Tuesday's figures.

Victoria also recorded one new infection acquired overseas, currently in hotel quarantine, bringing its total number of active cases to 81.

There are now 15,800 close contacts and more than 280 exposure sites, including sites on Phillip Island in the south of the state to the Mallee district in the northwest.

There were 54,839 test results returned during the 24 hours to Monday and 14,758 vaccines administered.

Australian Associated Press