Victoria’s chief health officer Brett Sutton has identified a worrying trend in the state’s growing number of Delta infections.

Professor Sutton said three-quarters of transmission of the virus is happening in people without symptoms, or before they have developed symptoms.

“We know it moves very fast. We know it causes really significant illness,” he said.

“What’s become apparent in recent weeks is that maybe three-quarters of transmission with the Delta variant occurs without symptoms or before you’ve developed symptoms.

“It is really critically important as well to minimise those contact, to not go out if you don’t have to and to absolutely wear a mask and maintain social distancing in all the circumstances that you can.

Victoria’s local COVID cases spiked to 80 on Thursday, prompting fresh alarm from authorities after a recent downward trend. They still say too many Victorians are waiting days to get tested, even after developing symptoms such as a fever.

“You have to assume that you could be infectious, even without symptoms, at any time,” Professor Sutton said.

“The idea that, you’re not in the right suburb, there’s been no cases here, there’s been no significant transmission identified around where I live, or where I work, it might be true today, but it may not be.

“You don’t know until it’s been identified. And this virus has moved silently and with stealth across Melbourne and into regional Victoria.”

Eighteen of the state’s new infections on Thursday are linked to the expanding outbreak in Shepparton, about 180 kilometres north of Melbourne.

None were isolating throughout their infectious period.

It brings the total number of cases in Shepparton to 67. The region has a population of 65,000 and about 17,000 are believed to be self-isolating, bringing the town almost to a halt.

Food distributors, supermarkets and pharmacies are among businesses forced to reduce operating hours due to staff shortages.

On Thursday, Premier Daniel Andrews said Victoria’s deputy emergency management commissioner, as well as other senior departmental officials, would head to Shepparton to coordinate relief.

“It is no different to a bushfire or flood. The emergency management architecture will be in place but it will be in the main very, very simple things – taking food to people’s doorstep, getting scripts filled,” Mr Andrews said.

“The focus is on getting everybody in Shepparton things they need when they need them.”

About 50 ADF personnel are already in the town to support Goulburn Valley Health with testing and door-to-door checks. Mr Andrews said a request for further support would be made.

Of Thursday’s cases in Victoria, 67 were linked to known outbreaks, while the source of 13 infections remained under investigation. Forty-one were infectious in the community.

Mr Andrews said several new cases had experienced COVID-19 symptoms for a week or more before getting tested.

“When you register a symptom, you cannot wait seven or eight days, as regrettably some of the positive cases did wait a long time before going to get tested,” he said.

“[It] meant they were in a community, out there unknowingly infecting other people and often the people they love the most.”

Mr Andrews urged Victorians to abide by stay-at-home orders, as there were “not many more levers” left to rein in rising case numbers.

“The longer people break the rules or make a judgment that the rules don’t apply to them, the longer these rules will be on,” he said.

“The more cases we have, the more cases that will be in hospital. That’s not being alarmist, it’s just being factual.”

There are 600 active COVID-19 cases in Victoria, including 36 people in hospital, with 11 in intensive care and eight on ventilators.

Meanwhile, about 200,000 Victorians booked in to receive a COVID-19 vaccine on Wednesday, after eligibility opened up to those aged between 16 and 39.

On a typical day about 30,000 vaccine bookings are made.

-with AAP