The number of dying sea birds turning up in the North Island are only a fraction of the total, which could number more than 350,000, a bird expert says.

More than 600 broad-billed prions were brought into Massey University's Wildlife Centre exhausted, starving and hypothermic when they arrived, and about half have died.

About 400 more birds were also taken into care in Wellington and birds were still being cared for by residents yesterday.

Sea bird ornithologist Phil Battley said members of the Ornithological Society of New Zealand had been making coastal searches to count the number of dead birds and had found between 400 and 1000 per kilometre. "This is only the ones that are visible on the surface."

Countless others could be buried under sand or submerged in the water, and others had been blown inland, he said. The huge number of dead birds was unprecedented.

Several situations had previously occurred – described by ornithologists as a "wreck" – but nothing on this scale, Mr Battley said.

Although the numbers of broad-billed prions worldwide were in the millions, those that had died in New Zealand could have a severe impact on their local population. There were only about a million of the species based around Fiordland, the Chatham Islands and Snares Islands.

"It's potentially quite a large proportion of the total population," Mr Battley said.

Ninety per cent of the dead birds found were broad-billed prions.

Te Papa's terrestrial vertebrates curator, Colin Miskelly, said prions were known to wreck in New Zealand, and the latest was the result of up to 10 days of strong winds. "Far more prions have been killed in this single event than the 37-year total recorded by the society."

Between 1960 and 1996, more than 86,000 prions were found dead on New Zealand beaches.

Mr Miskelly said he estimated about 350,000 of the birds would have died, and populations on some island could have been destroyed.

"Some of those [populations] might get absolutely knocked and take decades to recover," he said.

The broad-billed prions remaining at Massey University's Wildlife Centre were recovering, after losing their waterproofness being in captivity.

Centre director Brett Gartrell said they were slowly increasing their waterproofness, but they would not be released until late next week, because of wind and forecast rain.

Of these, 73 birds were left alive at the centre and they appeared to be doing well.

"The waterproofing is improving but very slowly," Mr Gartrell said.

"The ones that are left are looking much better and stronger and staying in the water, but still only for about 10 minutes."

After days of blue skies, when the birds were not strong enough or waterproof enough to be released, the wind hit hard yesterday.

- © Fairfax NZ News