The Yarra Riverkeeper Association says a bright green transformation of Melbourne's Yarra River on Thursday highlights broader issues around the lack of controls over litter in the waterway.
The Environment Protection Authority (EPA) is investigating the source of the spill involving fluorescein – a substance used by plumbers to check for leaks and blockages in sewage systems and underground pipes.
Melbourne Water said the discolouration was believed to have made its way into the river from the Elizabeth Street main drain and cleared up overnight.
"We are working with the relevant authorities including EPA Victoria to identify the source of the discolouration," the statement read.
The Yarra Riverkeeper Association said it was a similar substance to that which turned Venice's main canal bright green in May.
The substance is non-toxic, so should not have harmed the health of the river.
But riverkeeper Charlotte Sterrett said it should never have made its way into the water, which highlighted a problem with Melbourne's stormwater system.
"This issue points to a broader issue around pollution and stormwater, litter, chemicals, all sorts of things that come into the river," she told ABC Radio Melbourne.
"We saw last year with the floods, a lot of pollution in the river."
She said there was no one government agency responsible for managing litter in the river.
"So we see litter as a systemic issue," she said.
"Litter starts all the way up in the upper catchment and makes its way down to the river and out to sea and we're really only catching litter towards the end of the catchment.
"Anything we drop in the streets can end up in the river, whether that's chemical pollution, a cigarette butt, a plastic bag or piece of polystyrene."
The community organisation said it was awaiting a new report on the health of the Yarra River due later this year amid concerns among members that water quality was declining.
