The Department of Conservation (Doc) is seeking those responsible for using a chainsaw to hack out a a dead sperm whale's jawbone.
The carcass of the 15m whale, with a large cut around its jaw, was discovered on a beach north of Bull Creek, near Milton, on April 24.
Doc said adult sperm whales ranged in size from 11m-18m.
The theft has angered both conservation staff and local kaumatua (Maori elders).
Doc operations manager Annie Wallace said it was believed a chainsaw was used because it was the only tool which could cut through the huge bone.
"This act is pretty unusual because it would not have been an easy thing to do at all.''
The theft was an offence under the Marine Mammal Protection Act, she said.
"It is entirely illegal and we are taking it really seriously. It is something we are really unhappy about.''
About one-third of the bone was returned to the beach a few days after the discovery of the whale, she said.
"Our compliance officers will continue investigations to find the offender or offenders despite this recovery.''
Moturata whanau kaumatua Graeme Fraser said the runanga was concerned people "just attacked'' the whale.
"They didn't have any right to take it ... The bone which was recovered was being looked after by the Moturata whanau," he said.
Another kaumatua, Martin Palmer, said he thought the jaw had been taken for the whale's teeth.
"In the old days, if anyone found a whale like that the sailors would remove the teeth and make jewellery and trinkets.''
He was pleased some of the bone had been returned but said it would be better if the whole jawbone came back.
"We have to look after the remains.''
Ms Wallace said it was believed the whale died at sea.
Doc was liaising with the runanga on the disposal of the carcass.
The Bull Creek incident mirrors the taking of the jawbone of a porbeagle shark which washed up on Warrington Beach in late March.