Hundreds of oiled birds saved by a Wildlife Response Centre will be released soon, but about 20,000 birds may have died in the oil spill after the cargo ship Rena ran aground.
More than 2000 dead birds have been collected, but the estimated actual death toll, based on studies done in the US and England, is possibly 10 times that figure, Wildlife Centre manager Dr Brett Gartrell told NZ Newswire.
Only one to 10 per cent of birds in the studies wereever recovered, he said.
Marine predators or scavengers would have taken many of the birds, while others would have sunk to the sea floor, Dr Gartrell said.
The centre is preparing to release 409 birds - including 60 dotterels, four shags, and hundreds of little blue penguins - probably in two to three weeks' time.
Preparation involves ensuring the birds' feathers are waterproof.
"We put them in pools and we watch them and leave them for six hours, and then we check them to see whether they are repelling the water or getting wet to the skin.
"They need to be able to stay in the water for six hours and remain dry at the skin level."
Other work includes blood-testing and ensuring the birds' body weight is high enough to allow them to survive.
But Dr Gartrell said no birds will be released until there was no longer oil in the environment, and teams are checking it has all been removed.
The Rena ran on to the Astrolabe Reef off the coast of Tauranga on October 5, spilling about 350 tonnes of heavy fuel oil into the sea, coating local beaches and wildlife.