Victoria's COVID-19 testing commander Jeroen Weimar said on Tuesday authorities were looking at cases of "fleeting contact" between positive cases, in locations such as a Telstra Store in South Melbourne, a display home in Mickleham, JMD grocers in Epping and the Craigieburn Central Shopping Centre.

"It's not one or two, we now have at least four instances in this cluster of 54, where people who are unknown to each other before, during, and after have clearly transmitted to each other," he said.

"With previous variants, we are more used to transmission really occurring in the home, in the workplace or where people know each other, really having a lot of contact, or those big social settings."

The instances of very short contact led the Victorian government to urge anyone who had been at a list of shopping centres listed as exposure sites to get tested as soon as possible.

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President Elect of the Australian Institute of Occupational Hygienists Kate Cole said such cases showed how easily the virus could move in small places with no ventilation.

"In any sort of enclosed place like a shop, you're going to get a build up of aerosols," she said.

"If the shop doesn't have good ventilation, then other people can be exposed to those aerosols, they don't necessarily have to be standing next to them."

Ms Cole said businesses could check the ventilation in their spaces by using a simple carbon dioxide monitor, with readings of more than 600-700 parts per million showing stagnant air and increasing the risk of aerosolised transmission of the virus.

Measuring carbon dioxide was a "surrogate" for making an assessment on coronavirus, Ms Cole said.

"We're dealing with a virus that is transmitted through the air, it doesn't mean you have to be within 1.5 metres or there longer than 15 minutes, there are other factors at play."

Ms Cole said being vaccinated, wearing masks and keeping social distance all helped to lower the risk of spreading the virus, but that businesses also need to look at control measures for aerosolised transmission of the virus.

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