It's clear rodents caught in two Woolworths stores could pose a food safety risk.

The Dunedin South store has been closed since Friday, and 23 rats have been trapped, in the second infestation since late last year.

Meanwhile, Christchurch's Eastgate store is being investigated after video posted online showed a mouse on a salad.

Massey University Professor, Steve Flint, says mice and rats could contaminate food with salmonella or E. coli bacteria.

"There is some risk, whether it's a big risk or not depends on how many rats or mice are involved, how long they've been there for- and a whole range of other things."

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