Drainage engineers have traced the source of a discharge from an underground pipe into the Whanganui River.
A Chronicle reader sent in pictures of a green fluid coming from the small pipe which runs under the unsealed road between Landguard Bluff and South Beach near the city's airport.
The photos were taken on February 21, and the reader said there was a strong odour associated with the discharge.
Chris Carter, Whanganui District Council drainage engineering officer, said the dye was regularly used in the pipe network.
"The coloured dye is put down a drainage system to confirm where water flows to," Mr Carter said.
"A previously unknown pipe discharging to the river was discovered as part of the dye test. This has come from a pipe which isn't operated by the district council and our building staff are working with the owners of the pipe to make sure this problem is fixed."
He said the dye tests had nothing to do with checking for issues with the city's wastewater network.
"Dye tests are carried out on a case-by-case basis as part of one-off investigations.
"The frequency of testing varies, but it averages about one a month.
"The dye has no odour so any odour would be associated with the water in that particular pipe," Mr Carter said.