Waikato Regional Council has urged people to treat the bright orange discolouration of the Ohinemuri River as contamination.
The discolouration, which appears to enter the river downstream of the Waitawheta River, was reported to the council at around 3pm today.
Senior incident response officer Robert Isaac said the worst case scenario is that the discoloured water is due to a mine shaft collapse.
Old mines contain many heavy metals and contaminants, he said.
“Until we can ID the source we need to treat this as worst case scenario as we are getting reports of people standing in the water. If this is an old mine shaft that has blown out than people should not be using or touching the water.”
Other scenarios could include a collapsed forestry dam or other erosion.
Council staff are at the river and assessing the discolouration.