Otago Regional Council is warning people to look out for a potentially deadly algae in rivers and streams.
The algae, Phormidium, has been found in the Silver Stream near Mosgiel.
Phormidium is found on rocks or stones and is slimy and brown or black in colour.
Council resource science manager Matt Hickey says the algae is potentially fatal for dogs and in extreme cases death can occur within 30 minutes.
Mr Hickey says humans won't die from the algae, but medical attention should be a priority if any is swallowed.