A video posted to Facebook has shown a beautiful Taupō river being rapidly contaminated by an unknown substance.
The run-off, between Huka Falls and Reids Farm as the river flows towards Lake Taupō, was discovered by a local at around 11am on Tuesday morning.
"I had a day off work and thought it'd be a great day for a swim," Patrick Boyle told Newshub. "But I guess not."
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Mr Boyle said the discharge had covered most of the river, and was still freely flowing by the time he left 20 minutes later.
Waikato Regional Council is urging people to be cautious when using the river water after the muddy water poured in - although Contact Energy is adamant the discolouration is caused by "muddy water", and nothing more sinister.
The council says it is working with Contact Energy to determine the circumstances that caused the contamination.
Water supply intake has been shut down at Centennial Dr; this intake supplies both Centennial Dr and Rakaunui Rd, as well as the local golf club.
Caution is advised for landowners and businesses operating downstream.
A spokesperson for Contact Energy said it was investigating an incident that resulted in a slip, which flowed into a stream that feeds into the Waikato River. The Huka Falls have turned brown as a result.
"We are working with the local councils to assess the impact of this," said Contact's head of geothermal operations John Clark.
"We are conducting precautionary tests on the water and working with stakeholders in the Wairakei area to ensure they are informed."
A spokesperson for Waikato Regional Council confirmed the substance had originated from a Contact Energy holding pool, and that workers were on site.
"It's not effluent - it's a discharge that has caused brown discolouration," they told Newshub.