Thousands of seabirds are expected to die this summer as a result of unusual La Nina weather patterns.

The Department of Conservation (DOC) said penguins, petrels and other seabirds were already dying in large numbers and the summer weather was likely to cause the deaths of thousands of birds.

The La Nina weather pattern led to a reduction in fish stocks on which they fed, DOC vet Kate McInnes said.

Dead penguins had been washing up on northern beaches for several weeks and autopsies had shown they starved to death.

"Its not just penguins. All seabirds are struggling to feed their chicks or get into breeding condition as a result of the La Nina climate pattern," she said.

"It's sad but it's a natural event. It all points to a lack of small fish on which all these species depend to raise their chicks."

Grey-faced petrels, sooty shearwaters and gannets have all been affected.

Ms McInnes said the La Nina weather event was the biggest event of its type since 1975 and characterised by stationary anticyclones east of New Zealand bringing calmer weather in spring.

She said the calm seas reduced the feed for fish which went elsewhere, reducing the fish and food stocks for the birds.

Although many seabirds would die, they had excellent breeding last summer and population levels would cope.