Twenty rays potentially caught by fishers have been dumped at an Auckland bay, some with their tails chopped off, in what a marine scientist has called a "very cruel" act.
The Ministry for Primary Industries is investigating, and say the deaths could also have been caused by orca.
University of Auckland marine scientist Dr Shane Kelly came across the graveyard of stingrays and eagle rays on Saturday morning on a beach just south of Whakakaiwhara Point at Duder Regional Park, east of Auckland city.
Kelly and a team were out monitoring birds, and saw a bunch of black-backed gulls circling above the rays.
Kelly said they had been clearly caught by fishers, likely in nets, with visible marks on their bodies. Some had their tails chopped off.
"It was a bit of an unsightly mess, not a great way to treat wildlife."
The rays included some juveniles, measuring about a foot across, up to adults measuring around three feet.
The deaths and dumping were "clearly deliberate", Kelly said.
However he was not sure if it would have been commercial or recreational fishers.
Commercial fishers are legally required to land and report any bycatch.
Kelly said sometimes if rays were caught in nets they could be difficult to remove safely due to their stingers.
However with proper management they should be able to be released without killing them.
"So to find 20 or more all dumped together is really not on. Some had their tails cut off - it is a very cruel practice."
Rays were not uncommon in the Hauraki Gulf.
They play vital roles in the ecosystem, feeding off all sorts of creatures on the seafloor, and in turn are an important food source for orca - which are commonly seen in the Gulf.
"They are just fantastic creatures, very inquisitive," Kelly said.
"While they are common, even so our fisheries are under enormous pressure and this is the sort of thing we want to be managing effectively if we want to address the issues facing the Gulf."
Kelly was the lead author of the State of the Gulf 2020 report, which painted a grim picture of fisheries on the verge of collapse and sediment-choked and polluted waterways in Auckland and Waikato's big blue backyard.
"That report showed the Gulf has a lot of problems, so to see this kind of thing makes you wonder what the hell is going on."
Kelly had alerted mana whenua and the Ministry for Primary Industries about his grim discovery.
MPI acting national manager fisheries compliance Garreth Jay said it was not yet clear exactly how the animals came to be there.
Officers had examined the dead rays and taken photos.
Due to the way the tails had been removed they believed the deaths could have been from orca, which had recently been sighted in the area.
However, electronic monitoring showed there had also been commercial fishing vessels nearby recently.
"We will be looking into it to establish what happened," Jay said.
"We encourage anyone with information about this to share it with us through our hotline: 0800 POACHER (0800 476 224)."
The Hauraki Gulf Forum had also been alerted, a spokesman saying they were "very concerned".
"We deserve to know what happened to these beautiful taonga."