A Kaikorai Stream tributary in Frasers Gully glowed green on Wednesday night, possibly due to unconsented non-toxic tracer dye use.
Otago Regional Council environmental monitoring and operations director Scott MacLean said staff inspected the stream's discoloured water yesterday morning and identified remnants of tracer dye.
They were unable to tracethe source but suspected it may have been caused by a plumber checking drains, he said.
"Tracer dye is not toxic, will dissipate naturally, and does not harm people, animals or the stream environment."
Tracer dye discharge is allowed with a resource consent, but the council had not issued any for this activity.
It encouraged people to report any pollution to its pollution hotline on 0800 800-033 as soon as they saw it so staff could investigate, Mr MacLean said.
