You'd be forgiven for thinking some kind of water-borne nuclear attack was underway if you'd been standing on the Collingwood St Bridge this afternoon.

But the neon-green fluid spilling out of stormwater drains into the Maitai River wasn't toxic waste, but was a routine check of the Collingwood St drainage systems.

Some residents contacted the Nelson Mail concerned it was a chemical spill.

However Nelson City Council utilities manager Howard Schuppan said the dye, also known as fluocene, was a common tool that Nelmac used to track down cross-connections between stormwater and wastewater systems.

''It's part of an ongoing investigation we're doing to try and track down the source of E.coli readings in the Maitai River,'' he said. ''We're trying to improve the water quality.''

Monitoring had picked up higher than normal levels of E.coli in the river near the Collingwood St Bridge. ''If it's that visible to people there's possibly a reasonably big concentration, which would be because it's reasonably dry at the moment.''

The council last embarked on the exercise two years ago,and investigations were ongoing. ''The situation had improved somewhat but obviously there's still some areas we didn't catch last time."

The Nelson Fire Service attended because people had reported that it was a chemical spill.

Firefighters were shovelling stones to try and create a dam to block the dye from entering the Maitai.

Mr Schuppan said it wasn't normal practice for the council to inform the Fire Service of the operation, although he did tell Nelson City Council customer services.

- © Fairfax NZ News