Hundreds - potentially thousands - of little blue penguins have washed up dead on beaches along the east coast of the upper North Island in recent months.
The Department of Conservation said the mass die-off could be a one-in-20-year event that could partly be blamed on this summer's La Niña weather pattern.
Western Bay Wildlife Trust chairperson Julia Graham said it had received about 60 calls over the last two weeks about penguins that had washed up dead, or had died not long after they arrived on land.
Warmer ocean temperatures had meant there was a lack of food, or their food sources had changed, Ms Graham said.
The penguins were also trying to come ashore to moult at this time of year and if they didn't have enough fat reserves, they might not survive the two or three week process.
"They are coming out of it very, very weak and starving and then a combination of cyclones, offshore storms and bad weather has meant that they just don't have the energy to find food and fight the rough oceans anymore," Ms Graham said.
"By the time they're washing up, they're pretty far gone, they're emaciated, starving, no food, very, very exhausted."
Graeme Taylor from the Department of Conservation said they had had reports of penguins being washed up dead along the east coast of the upper North island, from the Bay of Plenty northwards.
This sort of mass die-off event happened once or twice every decade in moderate numbers, and once every 20 years in very large numbers.
"I think this may be the one-in-20-year cycle," Mr Taylor said.
"The last really big die-off occurred in 1998, when about 3500 penguins washed up on beaches. The sort of numbers we're hearing about could indicate we're up around that several thousand range."
But, Mr Taylor said, they were natural events.
"When birds have been checked in the past, they've all found to be dying from starvation, rather than from diseases or some other more worrying thing that could be causing mortality," he said.
People who find the little blue penguins on beaches are being told to leave them alone, unless they are obviously injured. Dogs should also be kept on leashes and under control.