The Victorian producer of the salads Tripod Farmers has been allowed to continue supplying its entire range to supermarkets after providing assurances to the Victorian Department of Health and Human Services that its plant was clear of contamination.

The confirmed cases of the rare Salmonella anatum​ strain, of which authorities would usually only see a handful of each year, has risen to 54 in Victoria, the department's senior medical adviser Dr Finn Romanes said on Friday.

Authorities launched an investigation after becoming aware of a spike of 28 cases of the rare infection on Tuesday, since then 26 more cases have been confirmed. Dr Romanes said Tripod Farmers had undertaken a "cleanup" of its facility on Tuesday night after some of its products tested positive to the salmonella strain.

"The identification of 26 new individuals who have unfortunately contracted this infection means that this is a serious outbreak," he said. "I think it's very likely that cases will still come to light and people will experience what's quite an unpleasant illness."

The department will interview the 26 new patients to determine whether they had also eaten any of the recalled products, which were sold at Coles, Woolworths, Bi-Lo and other grocers as Coles 4 Leaf Mix, Woolworths salad mix, SupaSalad Supamix and Wash N Toss salad mix.

Woolworths Gourmet mix 100g, Supa Salad ALH, 1kg, Clear Film Blend, Clear Film Spinach and Clear Film Cos were also added to the list on Thursday and Friday.

The contaminated products have best before dates up to and including February 14 and have been sold in all mainland Australian states and territories except Western Australia although the worst affected state by far appears to be Victoria.

A multi-jurisdictional outbreak investigation has been launched and all Australian health departments were scheduled to meet on Friday afternoon to identify the number of people infected and co-ordinate a response, Dr Romanes said.

Retailers Lite n' Easy and Aussie Farmers Direct have confirmed they sold some of the contaminated products, sandwiches sold at 7-11 are also believed to have been affected and Mentone Girls Grammar used some of the contaminated lettuce at its canteen on Thursday.

No pregnant women have been affected so far and no deaths had been linked to the outbreak, while only one child is among those who have fallen ill.

Tripod Farmers managing director Frank Ruffo​ said authorities had inspected its Bacchus Marsh processing plant and no further contamination had been detected. The company has returned to processing and distributing its full range of products, he said in a statement on Friday.

"Our complete production area, including floor and equipment, is sanitised daily and we conduct regular microbiological testing on our facility and product samples, in full compliance with Australian standards," Mr Ruffo said.

"We are a major producer in good standing in the industry and have been distributing washed and ready to eat products to the Australian and export markets for many years, without issue."

Fairfax Media was inundated with emails from people all over Australia claiming to be affected by the outbreak, including Doug Cresswell, a 52-year-old from Geelong, who described spending Thursday night in "excruciating" pain after eating salad bought at his local Woolworths.

"The pain that goes through your stomach and your chest, you feel like your chest was going to explode," he said. "I'm a fairly fit person and it's knocked me for six, I can only imagine older people how they'd feel if they'd been hit with it."

The strain of salmonella can come from contaminated eggs or chickens and the department has not ruled out that fertiliser may have caused the bacteria to spread among the vegetable crops.

Consumers are advised not to eat the recalled product, and to return it to the place of purchase for a full cash refund. If concerned, they should also seek medical advice from their doctor.

The full list of recalled products is available at www.foodstandards.gov.au.