Opawaho-Heathcote River Network is urging people in Christchurch to stop feeding bread to ducks.
"It isn't great for the ducks or our waterways," the group posted on its Facebook page.
Christchurch vet Pauline Howard said it was dangerous for ducks to be fed.
"Bread can cause developmental problems to their wings and discourages them from eating their natural food which is good for them," she said.
She said people did not necessarily realise it was bad for the ducks.