The regional council says prosecution is just one of many options to deal with polluters after paint ran through the Kaikorai Stream this week for the third time in two years.
On Wednesday, the Otago Regional Council said it was investigating after white paint was reported in the stream at the Nairn St Reserve. It identified painting contractors as the source.
Paint was found in the same stream last month and also late last year.
Last week, a Kaikorai Stream tributary in Frasers Gully glowed green, which the council believed was likely because it was contaminated with unconsented non-toxic tracer dye.
Regional council environmental monitoring and operations director Scott MacLean said he could not say how long the investigation of the recent paint contamination would take, as it would involve talking to many people.