The green waste dumped and leaching into Dunedin's urban streams is "offensive" and endangering the brown trout, Fish & Game says.
Fish & Game Otago operations manager Ian Hadland said some Dunedin residents were treating urban streams in Dunedin as dumping grounds for green waste, such as grass clippings.
When the streams flooded, the waste was picked up and dumped in Otago Harbour, he said.
"It's essentially littering."
Leaching from the compost could strip oxygen from the water and endanger the health of whitebait and trout.