The first little blue penguins caught in the oil from stricken container ship Rena are being released back into the ocean on Tuesday.

Sixty little blue penguins are being reintroduced to their original habitat after receiving treatment at the Wildlife Response Centre in Te Maunga.

Penguins recover at the treatment centre in Te Maunga.

They were taken to the centre following Rena's grounding on the Astrolabe Reef on October 5 as oil was leaking from the vessel.

National Oiled Wildlife Response team coordinator Kerri Morgan says birds will only be released after assessment of both the individual animals and their habitats.

'The habitats the birds are being released into have been carefully checked to ensure they are ready to receive wildlife.

'Penguins and dotterels are territorial and will return to the habitat they came from.

'It's important that we've removed as much oil as possible from their habitats before they are released.

'Each bird has been micro-chipped and the location they were found in noted – we will be releasing all the birds back to the habitat they came from,” says Kerri.

Each individual bird will undergo blood tests and other veterinarian checks to ensure it is ready for release.

All birds will also have to pass the ‘six hour test' where they swim for six hours without a break, before being assessed to ensure their waterproofing was returned.

'The oil coats the birds' feathers, which are designed to act as a waterproof coat.

'After the birds are washed, they preen themselves and that helps the feathers regain their waterproofing.”

Birds that are given a clean bill of health for release are also re-introduced to salt water.

The pools the birds are swimming in are fresh water, but to get the animals ready to return to the sea, salt is introduced into their pools over several days until they are swimming in water with the same salinity as the sea.

'We have been going out with the oil spill response teams for the past week or so to check that the places we want to return them to are ready.”

Kerri says although the risk of a further spill of the residual oil onboard Rena is still there, this risk has to be balanced against the risk of keeping the birds in captivity for too long.

'These are wild animals and they belong in the wild. We know there is still a chance that more oil may spill from Rena – but we don't know when and we don't know where that might wash up.

'We can't keep wild birds in captivity for an indefinite period of time without running the risk of disease or injury.”

Kerri says the first 60 penguins are being released on Tuesday and she hopes more will be released later in the week.

'We still have birds that need to finish waterproofing, so the release programme will take a while yet.”

The wildlife facility at Te Maunga will slowly be dismantled as the cleaned birds move through the washing, re-waterproofing and salt water process and become ready for release.

'We will maintain a few permanent structures there until Rena is off the reef and there is no longer any risk of an oil spill from the wreck.

'That way, we will be ready to rebuild the facility and mount a response if needed.”