Morisset Street and Collett Street in Queanbeyan were closed Thursday as fire crews fought to contain a suspected petrol spill in the Queanbeyan River.
NSW Emergency services launched at least two boats into the river near the Art Society Gallery and a riverside cafe.
Authorities suspect the chemical spilled was fuel.
"Currently we've got a hydrocarbon, be it petrol or diesel, leaking into the drainage system coming down and entering the river way," Queanbeyan Station Officer Wayne Huggins said.
He said winds were not helping in the battle to contain the spill.
"The surface of the water has a sheen due to the oil on the surface and it's currently going upstream due to the wind blowing it up."
Fire and Rescue crews were at the scene and were using floating barriers to keep the substance from spreading.
"We're going to put a barrier that floats just around the front of the drain pipe and with the assistance of the SES we'll be putting up a bigger one to stop spreading the stuff actually in there. Then what we'll do is drag that back and skim most of it off the surface," Mr Huggins said.
The crews brought one pumper, one commander, the Hazardous Material Unit and the Breathing Apparatus Van.
Nigel Sargent, manager of the south east region of the Environment Protection Agency, said significant fines, up to $15,000 for a company and $7,500 for an individual, applied to anyone caught deliberately contaminating a waterway.
He said hydrocarbons in stormwater can cause environmental damage and impact adversely on any ecology or animals that come into contact with it.
With the Queanbeyan Age