The Otago Regional Council investigated after the Water of Leith ran orange for much of Saturday and Lindsay Creek turned purple.
It soon became clear someone had been putting dye into various spots in both waterways. Council staff found a bag of dye in the Leith and a member of the public found another at Lindsay Creek.
Council environmental monitoring and operations director Scott MacLean said no-one had notified the council they were carrying out dye testing and the ORC had come to the conclusion a prankster was responsible.
Putting dye in the streams was "not a very clever thing to do" and potentially against the law, Mr MacLean said.
"They don’t necessarily know what effect the substance they are putting in the stream might have on the ecology."
It was also upsetting for members of the public who came across it.
There was nothing to suggest the dye had caused harm to wildlife in the stream, but council staff would check again today.
Amelia Seales said she noticed parts of the Leith near Woodhaugh Gardens were orange on Saturday afternoon when she was walking her dog.
"I was about to let my dog have a drink ... and we were like ‘You are not touching that’."
She first noticed the odd colour at 1.30pm and it was still there at 6.30pm.
The dye had a "chalky" consistency in the river and looked as though it could be toxic.
"It almost looked like you could grab it and pick it out of the water."
She said she called the Otago Regional Council and was told a prankster or pranksters were responsible.
"Apparently it’s to make the waters of Dunedin more colourful."
