A 8 km long oil slick has contaminated Melbourne's Yarra River, posing serious threat to wildlife in the region.
Recreational users have been warned against contact with the water until the slick - from the Chandler Highway to Johnston St - is cleaned up.
The Environment Protection Agency (EPA) is investigating the source of the spill which, they believe, happened last Saturday but was not discovered until Tuesday by a kayaker using the river.
EPA regional services executive director Bruce Dawson said the spill had caused a light sheen to settle on top of the water.
He estimated about 100 litres of oil had leaked into the Yarra.
The slick stretches along a section of the river that flows through Yarra Bend Park, a popular recreational area.
The park is home to more than 180 native species, including 13 of the state's threatened species.
Birds commonly found in the park include the rainbow lorikeet, red-rumped parrot and on occasions, the yellow-tailed black cockatoo.
Mammals include echidnas, native water rats and brushtail and ringtail possums.
"The major potential risk is wildlife or animals coming in direct contact with the oil. It can get on the feathers of birds if they dive into the water," he said.
An absorption boom has been placed across the river upstream of Dights Falls to soak up the slick, and no wildlife have been injured so far.
He said the oil below the boom would dissipate before it reached Port Phillip Bay.
"The river is not closed and there's no restrictions to people using boats, as long as people don't come into direct contact with the water," he said.
"Don't let it come into contact with your skin."
The EPA is working with staff from Melbourne Water and Parks Victoria to clean up the spill and collect samples of the oil.
"We are testing samples to narrow down what type of oil it is and to determine if it's new or old, automotive or industrial," he said.
"It will prove very difficult to find the source of the material. We are analysing the samples and have people in the field trying to eliminate where it came from."
Dawson said that there was a large network of stormwater drains through which the oil could have flown into the river.
"It's going to be a significant challenge to determine precisely the source as there are many thousands of possible sources of the oil," he said.
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Even otherwise storm water flowing into Yarra was threatening its health, Chris Walsh, an expert commented.
The Yarra River Action Plan, launched by the State Government 18 months ago, earmarked $250 million to deal with contamination from leaking septic tanks and $20 million to target storm water pollution.
In 2005, an investigation by a prominent newspaper, The Age, revealed widespread pollution in the river, including high E.coli levels. Recent EPA monitoring found poor water quality at several city sites.
Dr Walsh said stormwater contained "a cocktail of polluting substances", including cigarette butts and chemicals.