The habitat of cane toads is spreading across Australia and the poisonous amphibians are decimating populations of freshwater crocodiles, according to a leading biologist.

Dr Mike Letnic, of Sydney University, says scores of crocodiles in the tropical, northern region have died after eating toads. His team visited the Victoria River district of the Northern Territory.

"[In 2006] we counted more than 600 crocodiles and in 2007 we counted less than 400," he said. "There were dead crocs everywhere. The only thing that had changed between visits was that cane toads had moved through the river system."

Letnic said examination of the crocodile carcasses showed they had eaten cane toads. "Crocs don't ask questions when looking for a meal," he said. "We think there were big concentrations of toads."

Letnic, who presented his research at the Australasian Vertebrate Pest conference, said the small size of freshwater crocodiles made them vulnerable as they feed more often, and on smaller prey than the larger saltwater species.

Cane toads were introduced from South America to combat beetles infesting sugarcane. They had little effect but bred prolifically and proved an environmental peril, with their toxin killing native animals.