Hamilton City Council is investigating an unintended discharge from the city's water treatment plant.
Chris Allen, Council's City Infrastructure General Manager, says the discharge occurred in late December, and early investigations suggest a mechanical failure has been the cause.
"Approximately 650 m3 of Alum sludge was discharged to the plant's onsite storage system, where it mixed with process water that is discharged to the river," Mr Allen says. "We believe that, due to this mixing, environmental impacts will have been minimal."
Alum is a chemical used to remove naturally occurring elements like arsenic from drinking water.
"We're extremely disappointed, and we apologise to river users and Waikato Regional Council, which is the consenting authority for river use. This should not have occurred."
Mr Allen said Hamilton City Council was working closely with Waikato Regional Council on the investigation, and would not comment further while it was underway.