The number of mutilated Hector's and Maui's dolphins found on the coastline has more than tripled in the past three decades, Department of Conservation statistics show.
Data gathered under the Official Information Act, showed there were five cases from the 1980s, 14 cases from the 1990s and 17 cases since 2000.
Five dolphins had their bellies slit open, one in 2004 had its head cut off and had been hit with a blunt instrument, eight had their tails or tail flukes cut off, two had fins cut off, two had been filleted, one was gutted and another nine dolphin carcasses had knife marks on them.
In one case in 1990 a dolphin skull was found that showed the dolphin had been shot, with clear entry and exit wounds, and in another case, in 1989, a carcass was found with a partially severed head that had probably also been shot.
In 15 cases the dolphins were known to have been killed in set nets or had set net marks on their bodies.
Forest and Bird marine conservation advocate Kirstie Knowles said it was feared the mutilations were inflicted by fishers who had accidentally caught hector's dolphins in their nets and were trying to hide the evidence by cutting up and burying the dolphins or letting them sink.
"It is not possible to definitely attribute the mutilations to any individuals or groups, but the fishing industry is keen to avoid tougher measures to protect dolphins from being tangled in fishing nets."
Hector's dolphins are an endangered species and Maui's dolphins are critically endangered with just 111 individuals left.
In May the Government announced measures to protect hector's and maui's dolphins.