E.coli has been found in popular swimming holes on the Taieri River in Central Otago.
Otago Regional Council says warning signs have been put up at swimming spots alongside the Otago central rail trail, between Creamery Road and Waipiata near Ranfurly, warning swimmers they could become ill if they swallow the contaminated water.
Council director of engineering, hazards and science Gavin Palmer says staff have tracked the river through nearby farms looking for the source of the contamination but haven't found any cause of the E.coli.
Further inspections and water quality testing are planned this week.
Dr Palmer says farmers have been asked to keep a watch on stock near waterways.