This has been a disastrous season for little blue penguins, and the birds are still dying in droves.

Pakiri Beach north of Auckland was littered with dead penguins last weekend, something long-time resident Hilary Russell had never seen before.

"We couldn't believe it.

"You see the odd one from time to time, but nothing like this."

Sick and dying birds have been seen around the North Island, particularly the Bay of Plenty, Coromandel Peninsula, Hauraki Gulf and Northland.

Bird rescue workers have been kept busy since early summer trying to save them.

North Shore's "bird lady" Sylvia Durrant, of SPCA Birdwing, has had 140 brought in by the public since early summer.

"I usually only get a few babies, maybe 30 a year, so something has gone wrong out there," she said.

"All the ones brought in are starving."

The latest round of deaths appeared to be caused by the annual three-week moult. Because the birds don't go to sea to fish while losing their feathers, many starved to death without the normal fat reserves to see them through, she said.

"I got the babies in December because their parents had stopped feeding them. I got the adults in January who were so thin they were dying, and now I'm getting the ones that are starving during the moult," Mrs Durrant said.

Scientists' best theory is that the mass deaths occur naturally about once a decade, caused by a combination of not enough food and rough weather.

"It seems to be a natural event about once a decade if you look at the records, which go back 40 years," said Department of Conservation scientist Graeme Taylor. "But populations are robust."

A pattern of sea currents and warm temperatures may have prevented tiny sprats, the penguins' main diet, from spawning in high enough numbers or rough weather could have made it difficult for the birds to fish, he said.

The little blue penguin is a protected native species and found throughout coastal New Zealand and southern Australia. They fish during the day and come ashore at the same beach each night, sometimes living under beachside homes.

Little Blue Penguin

* Found around New Zealand and in Australia, where they are known as fairy penguins.
* Often roost for the night under seaside houses.
* World's smallest penguin, only about 25cm tall.
* Have been known to dive more than 60m in search of food.
* Can live for up to 25 years.