The Ohinemuri River north of Tauranga has turned bright orange.
Hauraki District Council said it was aware of the issue turning the water in the river orange.
“This is likely from water coming out of old mine workings,” it said in a post on Facebook.
“Waikato Regional Council is investigating the issue and advise for safety reasons, please do not enter the water.
“Our Paeroa water supply intake is upstream of this so is not affected however we’ll be monitoring the situation carefully.”