The Southland Regional Council is warning locals to keep their dogs away from the Winton Stream, after a significant amount of effluent was discharged into a tributary.
The council is investigating the discharge and has advised people not to go into the stream, or allow dogs to drink from it.
Farmers who drew water from the stream or its tributaries were being advised to make alternative arrangements.
The council said it hoped to give the all-clear for water from the stream early next week.
It said the Winton water supply was not affected, and was safe to drink.