New Zealand animal experts will begin releasing seabirds Tuesday almost seven weeks after a massive oil leak from a grounded cargo ship triggered a wildlife rescue operation.

Over 300 Little Blue Penguins are in care at the Rena Oiled Wildlife Recovery centre. [stuff.co.nz]

Maritime New Zealand (MNZ) announced Monday the staged release of some of the cleaned birds from the oiled wildlife facility in the Bay of Plenty region would start this week.

All birds would have to pass a "six-hour test" where they swam for six hours without a break before being assessed to ensure their waterproofing was normal. They had also been introduced back to salt water.

The habitats where the birds would be released had been carefully checked to ensure they were ready to receive wildlife.

About 350 tonnes of oil leaked from the Liberian-flagged Rena after it ran aground on the Astrolabe Reef on Oct. 5, killing an estimated 1,300 seabirds.

More than 400 birds are being cared for at the oiled wildlife response center, including 60 New Zealand dotterel that were captured pre-emptively to protect the endangered species.

Although there was still a chance that more oil could spill from Rena, the center could not keep wild birds "in captivity for an indefinite period of time without running the risk of disease or injury," said Morgan.

The first 60 penguins would be released Tuesday and it was hoped more would be released later in the week.