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.
Prosecution was one of ''many tools'' the council could use in response to the pollution, he said.
The previous paint contaminations did not lead to prosecution, because the sources were never identified.
"It can be quite difficult to find the origins of it."
Such pollution was a "real concern" for the council, he said.
"We're also preparing a piece of work on urban water quality. Part of that is raising awareness and making people mindful of what goes down stormwater drains.
"It doesn't just go out of their sight, inevitably it ends up in a river or the harbour."
Kaikorai man Pete Andrews said he saw contractors in the area on Wednesday washing paint into a stormwater drain.
"I came around the corner and I could just see the white paint everywhere."
The contractors were water blasting buckets, brushes and rollers, he said.