A popular waterway near the Banks Peninsula town of Akaroa is being described as extremely toxic.
The Canterbury District Health Board has issued a health warning, saying an extreme toxic contamination has been found in Walnut Creek.
The Canterbury Regional Council said dead eels have been found, but it does not yet know what killed them.
Medical Officer of Health Alistair Humphrey said Walnut Creek is a popular site for gathering food such as watercress and eels.
Dr Humphrey advised people not to eat or drink anything from the creek until the health warning has been lifted.
The council is determining the source of the contamination.