Fourteen native seabirds were found mutilated and dumped near Ruakaka in October, according to Forest and Bird.
The 13 flesh-footed shearwaters and one sooty shearwater reportedly had their throats slit, skulls smashed and wings broken.
There was evidence the threatened birds had been hooked and entangled in fishing equipment.
Forest and Bird says the birds may have been caught in a beach-based long line. The way the birds had been dumped indicated they had been poured out of a fish bin.
Forest and Bird says the killings are "barbaric and completely unacceptable".
Flesh-footed shearwaters recently had their conservation status updated to Near Threatened by the International Union for Conservation of Nature (IUCN) due to their high risk of dying at the hands of commercial and recreational fisheries.