As of midnight on Wednesday, everyone in Victoria's lockdown regions of metropolitan Melbourne and Mitchell Shire must wear masks when they are outdoors.

They risk a $200 fine for not following the new rule, while people in regional Victoria are also being encouraged to wear masks as a precaution.

But Prof Sutton noted that there will be exemptions.

"A number ... are legitimately not able to wear masks so please don't vilify individuals or don't make the assumption they are simply stubborn," Prof Sutton said.

"There will be people with medical, behavioural, psychological reasons ... certainly don't make an assumption that they should be the subject of your ire."

Supermarkets have made it clear they will take a hard line with customers on the new rule.

Woolworths, Coles, Aldi, FoodWorks and IGA all feature in an advertisement about face masks.

"We expect all customers to be wearing a face covering in our stores, unless a lawful exception applies to you," the ad reads.

"Masks have also become compulsory for our team members, unless they too have an exception."

Premier Daniel Andrews said on Wednesday that supermarkets are an excellent example of why masks are necessary, as Victoria tries to reduce the rate of transmission.

Wednesday's figure of 484 new cases is an unwanted record and continues a run of triple-digit days that started on July 6.

Given supermarkets are indoors and they have so many customers, health experts are particularly worried about community transmission.

"That's exactly the sort of setting where wearing masks will be critically important," Mr Andrews said.

"And you may well be turned away if you turn up ... and you don't have a mask on."

The premier also scoffed at reports of people selling masks for use by babies or toddlers.

Children under 12 do not have to wear masks.

"That is simply not on. Toddlers should not be wearing masks," he said.

Australian Associated Press