A conservation group on Queensland's Sunshine Coast says the region's waterways have been hard hit by flooding in the past month.
The CEO of Maroochy Waterwatch, Cerran Fawns, says heavy rain has caused a lot of landslips in the upper catchment of the Maroochy and Mooloolah rivers.
Ms Fawns says when combined with riverbank erosion, a large amount of silt has washed into the ocean.
"The soil particles when it's a really hot day get super heated and if you've got really hot water ... then you are at risk of fish kills," she said.
"What's more concerning is the amount of debris that has been washed down.
"When you've got a lot of organic material in the waterways it breaks down over time and that breaking down process rips the oxygen out of the water, so you can actually be at risk of fish kills from that process as well.
"Basically a lot of tree roots and twigs and branches ... and there's the odd bit of large material like fridges and mattresses and the usual litter suspects but mainly the bigger vegetation matters that are concerning."
© ABC 2013
12:06 EDT Transition season is well under way across northern Australia with thunderstorms a regular occurrence each afternoon.