Contractor responsible for Arrowtown's neon green river

Otago's fresh water watchdog has given the all-clear after being tipped off by a member of the public that a section of Arrowtown's usually clear river was flowing bright green.

The Otago Regional Council's compliance manager Tami Sargeant says it received a complaint via its Pollution Hotline on Friday.

"An incident responder checked the Arrow River then and the river was clear at the time they were onsite. We’ve since identified that a contractor was undertaking investigation within the area using tracer dye, which is the likely source."

Ms Sergeant says research suggests tracer dye is not toxic to humans and has very little effect on aquatic ecosystems as it decomposes quickly when in contact with sunlight and air.

Photographs of the tracer dye in the river circulated on social media on Friday, after the unusual looking water was spotted by a local walking their dog.

It is not the first time the bright-coloured tracer dye has stopped people in their tracks. 

Two years ago Queenstown's Horne Creek turned neon green, after large amounts of the same tracer dye, generally used by plumbers, was believed to have been put into the waterway as a prank, sparking an investigation by the Queenstown Lakes District Council.