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.

He says the level of risk depends on how many animals there are - and how long they've been there.