Department of Health deputy secretary Jeroen Weimar said Bendigo primary contacts had been contacted.

"We saw some minor detection and Bendigo in wastewater," he said. "We have contacted primary contacts in that area.

"We have some testing coming up and we are doing some extra sampling today to see if there is more to be concerned about. If so, we will make an alert."

There are now 78 active cases of coronavirus in Victoria.

Acting premier James Merlino said the restriction easing plans would continue as outlined on Wednesday with the exception that masks would be mandatory outdoors in Melbourne.

"Based on public health advice, we will continue to take those cautious next steps from 11:59pm tonight," he said.

"This is not over yet and the risks of this virus will be a factor in our lives in the long term and in the short term so in the short, that means we are moving ahead but we are on high alert and the public health advice is very, very clear.

READ MORE: New coronavirus restrictions for regional Victoria explained

"We can only consider doing something like this based on continuing high-test numbers like the 23,679 tests that we recorded yesterday; that's a strong number and I want to thank everyone who came out to be tested yesterday, but given the record numbers over the last week or so, I think that we can do better.

"That's the very thing that is giving us the clear picture to provide public health with the advice that it has in order for us to ease restrictions so this absolutely must be maintained if we are to make those cautious next steps in a week's time."

The four new coronavirus cases were all locally acquired but were all from the same household in Melbourne's northern suburbs and investigations into acquisition are underway.

On Wednesday night, the Health Department revealed that viral fragments were found in a "sewer sub-catchment serving suburbs north-west of Bendigo".

The department urged residents of and recent visitors between June 3 and 7 to California Gully, Eaglehawk, Epsom, Huntly, Jackass Flat, Maiden Gully, Marong, North Bendigo and Sailors Gully to monitor for symptoms of COVID-19 and get tested if any symptoms develop.

Bendigo Health is working closely with the Department of Health and urged people in affected suburbs not to panic but monitor for symptoms and get tested if symptoms develop.

Bendigo's Stewart Street COVID-19 Screening Clinic is open seven days, from 10am - 5.30pm.

On Wednesday, acting premier James Merlino announced restrictions in Victoria would ease further from 11.50pm on Thursday night.

More detailed information is available at dhhs.vic.gov.au/wastewater-testing-covid-19 and the Department of Health is updating the exposure list to 180 sites via coronavirus.vic.gov.au/exposure-sites.

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