The Queensland Fire and Rescue Service says there has been widespread chemical contamination of the Burnett River due to flooding.
Inspection crews have been visiting farms in the North Burnett to identify and collect hazardous material washed away in floodwaters.
It includes chemicals and fuel from farms and much of it has washed across farmland.
Inspector Ray Bott says it is hard to quantify the extent of the contamination.
"We couldn't measure anything but from what I've seen going down the river, it was a lot of chemicals and materials and there were a lot of drums that we saw that have washed up so that sort shows the extent of materials in the river," he said.
"There were pesticides so they can be toxic to the water animals and there were a number of oils as well."
Authorities now fear stretches of water will lose oxygen, killing fish and increasing acidity.
Meanwhile, a hydraulic engineer says the Queensland Government could install floodgates on the Paradise Dam to reduce future flooding of the Burnett River.
The Government is reviewing how dams performed during the recent floods and whether they can be upgraded to provide greater protection.
Max Winders studied Paradise Dam for a High Court case.
He says it would be possible to upgrade it, so it could release water earlier and reduce flooding.
"The Paradise Dam is the best place to start," he said.
"It's a recently constructed dam and it's constructed from reinforced concrete and it demonstrated it's strong enough to take quite a substantial flood."