Otago Regional Council is warning water users to avoid the confluence of Dunstan Creek and Manuherikia River today after toxic blue-green algae was discovered in the area.
The confluence is near the Central Otago settlement of Becks, about 50km northeast of Alexandra.
Existing toxic algae alerts are also in place for the region's Cardrona River.
The council said the algae could produce toxins that are a risk to human and animal health.
Meanwhile, residents in the small Banks Peninsula town of Diamond Harbour are being asked to boil water after E coli was found a water reservoir during routine water testing yesterday.
Christchurch City Council's head of three waters and waste John Mackie said the reservoir only services a handful of properties, but a boil water notice was put in place for the town's 1100 properties as a precaution.