The Department of Conservation says post-mortems on 18 little blue penguins found on two beaches on the East Coast show they died of starvation and exposure.
A spokesperson, Jamie Quirk, says both adult males and females were examined by veterinarians at Massey University.
He says they concluded that 12 of the penguins died due to a combination of starvation and being emaciated, with exposure being an added cause of death for another five penguins.
Mr Quirk says the deaths are linked to La Nina weather patterns which reduce the population of baitfish penguins feed on.
Earlier, a Gisborne District councillor linked the deaths to seismic testing off East Cape by the Brazilian oil company Petrobras.