Waikato Regional Council is investigating a foamy substance seeping into the Waikato River from Hamilton's stormwater system.
It's now urging people who take water from the river immediately downstream of the site to exercise caution until the substance is identified.
Concerned members of the public first contacted the council about 11am Sunday after spotting the discharge from a pipe at the northern end of the golf course in St Andrews.
While the substance appears to be detergent-based, there are a number of samples which have been collected by staff for analysis. Results are expected to be available tomorrow afternoon.
In the meantime, a council spokesperson said it was working with Hamilton City Council staff to work out where it has come from.
So far, the stormwater pipe had been traced to the Te Rapa industrial area.
Any members of the public with information which might help investigators are urged to contact Waikato Regional Council on 0800 800 401.