At least five protected fluttering shearwaters may have been shot dead near Little Barrier Island in the Hauraki Gulf.
The Department of Conservation said the dead birds were seen floating on the water by an Auckland man, who said they appeared to have bullet holes in them.
Anyone who may have seen what happened is being urged to contact DOC.
Fluttering shearwaters are protected under the Wildlife Act.
DOC said people who harass or shoot them, or any other protected species, could be prosecuted.
The penalty for doing so was a jail term of up to two years or a fine of up to $100,000 dollars.
In 2015, four takahe - which are endangered - were shot on Motutapu Island in the Hauraki Gulf by hunters, who misidentified them during a pukeko cull.