An absorbent boom has been placed across the Emu River at Burnie to contain an oily film on the surface of the water.
Environment Protection Authority Director Warren Jones said the contaminant creating the film was believed to be hydrocarbons and had been traced to stormwater draining from an industrial area of the suburb of Wivenhoe.
Mr Jones said the EPA and the Burnie City Council were notified about a slick on the river on Friday afternoon.
"The Council had a crew on the ground attempting to trace the contaminant to its source," he said.
"The EPA and the Council have been investigating stormwater issues in the Wivenhoe area for some time.
"Tests on water samples have so far been inconclusive and council environmental officers have continued to monitor the situation."
Mr Jones said the boom across the river would trap the contaminant before it entered Bass Strait.
He said South Burnie and Wivenhoe beaches had been checked and were clear of the contaminant.
The EPA will liaise with a wildlife care coordinator about the welfare of domesticated ducks on the river.