Three people are infected so far in the Shepparton outbreak, with 400 people isolating and about 4000 tests in the local community over the last two days.

So far, about 350 tests have all come back negative.

The infected driver failed to tell authorities he had visited the northern regional town first quizzed about his movements and has been referred to Victoria Police.

Victoria Police confirmed on Thursday morning it will assess whether any criminal offences were committed.

By the time the driver's connection to the Chadstone Shopping Centre outbreak was becoming clear to authorities, he had already left on his September 29-30 trip to Kilmore and Shepparton with a permit.

"There is unfortunately ... that slipping through the net. A lot of the spread had occurred before (he) became symptomatic," said Professor Cheng, the deputy chief health officer.

The truckie admitted to illegally dining at a Kilmore cafe and visiting Benalla.

But he only told DHHS contact tracers he stopped in Shepparton after three people tested positive earlier this week.

DHHS testing boss Jeroen Weimar also revealed on Thursday that the driver stayed overnight in Kilmore on September 29.

Everyone in Shepparton has been urged to undergo testing.

Premier Daniel Andrews said there 1862 tests in Shepparton and nearby towns on Wednesday - usually the daily number had been around 60.

He added there were about 400 people isolating in the Shepparton community who are either outbreak cases, contacts or their contacts.

"They are isolated at home and they are isolated at home because of one person. That's the nature of this virus," he said.

DHHS testing boss Jereon Weimar said 350 Shepparton test results from Wednesday had all come back negative.

About 2000 tests had been done so far as of Thursday morning.

One testing site at Shepparton Sports City had reached capacity soon after it opened at 10am on Thursday, but a second drive-through facility was due to open at that location from midday.

Mr Andrews apologised for the testing delays, but also defended the process.

"There's absolutely no notice provided here. That's the challenging part of it," he said.

"In less than 24 hours we have been able to stand up 11 test sites, if there needs to be 12, 13, 14 ... (but) there are limits on how quickly you can stand them up."

The three people who tested positive in Shepparton worked at Central Tyre Service on Welsford Street between September 30 and Tuesday.

Another five sites are also considered high-risk, with a further four listed as locations of concern.

Chief Health Officer Brett Sutton has "no doubt" the Shepparton community will get on top of the outbreak.

Victoria reported six new cases of coronavirus but no new fatalities, leaving the state's death toll at 816 and the national toll at 904.

Melbourne's rolling 14-day new case average has dropped to 8.9, and remains steady at 0.6 for regional areas.

Department of Health and Human Services figures also show the number of mystery cases in Melbourne rose by one to 15 for the period September 29 to October 12. There are no mystery cases in regional Victoria.

Also, police are investigating after Mr Andrews' electorate office was vandalised for the second time in less than a month.

His Noble Park office was cordoned off after "Sack Dan" was spray painted on the office and one of the windows cracked, apparently by a thrown brick.

Australian Associated Press