Dead seabirds washing ashore along the Bay of Plenty coast and north of Gisborne starved to death, experts say.
The broad-billed prions (Pachyptila vittata) are normally found off the Otago coast, Chatham Islands and lower Tasman Sea, where upwelling of currents provide a rich source of plankton on which they feed.
Necropsies on the dead birds were carried out by Massey University vets.
Department of Conservation ranger Anastacia Kirk said it was likely the prions had been pushed out of their feeding grounds by strong southerly winds and, once away from their normal feeding area, had become fatigued, emaciated and unable to return south.
"Two days of strong northerly winds averaging 65km/h in the Bay of Plenty may have pushed these birds inland.
"Their efforts to avoid land may have weakened them further."
A mass death of prions occurred on the West Coast of the North Island last year when 250,000 birds died in a severe winter storm. This was the largest recorded beach death of seabirds in New Zealand.
Graeme Taylor, seabird specialist at DOC, said there were an estimated one million breeding pairs of broad-billed prions around New Zealand.
"It remains to be seen if last year's storm event, and the recent East Coast event will affect the population in the long term."
Beach users can record any dead birds they encounter on the Ornithological Society Beach Patrol Scheme website.
This information will contribute to the knowledge of seabird distribution, abundance, and the cause of seabird deaths on New Zealand coasts.