Australians have been assured pre-packed lettuce on retail shelves is safe to eat as the number of salmonella cases linked to some products grows.

The number of people infected with the rare salmonella anatum strain has reached 74, although numbers are expected to start tapering out.

Pre-packaged lettuce distributed by Victorian-based company Tripod Farmers was recalled on Thursday after the salmonella outbreak.

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But Fresh Produce Safety Centre technology manager Richard Bennett said consumers shouldn’t worry about potentially tainted lettuce unless it had been sitting in their refrigerators.

“Any product on the retail shelf now is fresh, safe and healthy,” Mr Bennett told AAP.

“It (salmonella case numbers) could have doubled overnight but it didn’t, which tells us the health authorities and retailers have been swift and effective in their recall.”

Mr Bennett said more cases of the rare salmonella strain were possible, although numbers would start to taper down given the infection period was a maximum 72 hours.

So far, cases of salmonella anatum have been recorded in Victoria, Queensland and South Australia.

Investigations are under way to determine whether a contaminated water supply, fertiliser or something else could be to blame for the outbreak.

Mr Bennett said Australia was a leader in food safety and systems were usually able to prevent outbreaks of disease.

The recall is for 30 retail products sold at supermarkets, as well as wholesale products.

The recalled lettuce mixes with any best-before date up to February 14 should be returned to the store, or thrown out.