Life rafts are stopping rare native gull chicks from drowning in a flooded Christchurch ruin.

About 300 critically endangered black-billed gulls/tarāpuka, the most threatened gull on earth, are nesting on the concrete beams of a fenced off, half-demolished office block on Armagh St, and mating and washing in the flooded basement.

Department of Conservation (DOC) got involved after members of the public reported seeing chicks falling into the water and becoming stranded. About two chicks are thought to have drowned.

JOHN KIRK-ANDERSON/STUFF An adult black billed gull with chick on the raft, with red billed gulls around.

DOC staff on Wednesday evening used kayaks to install four 1.2 metre square "life rafts" made of wooden pallets and 20-litre containers, so newly-hatched chicks falling from their nests could clamber to safety. They will then stay on the platform until they fledge.

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DOC ranger Vanessa Mander said when the chicks hatched, they creche together to keep warm and protect each other; "This isn't ideal behaviour when your home is a 1m wide concrete beam".

JOHN KIRK-ANDERSON/STUFF Most of the chicks are yet to hatch.

"While the fence and the water surrounding the colony has helped to prevent predators such as cats and rats from eating eggs or attacking the chicks, this waterlogged building site with little room for a growing colony has proven to be far from ideal.

"The significantly larger karoro/black-backed gulls have also started circling and picking off vulnerable chicks."

The colony is thought to have come from the Avon Heathcote Estuary, as birds that generally lived there over spring did not show up this year. Most of the chicks are yet to hatch.

JOHN KIRK-ANDERSON/STUFF Black billed gulls nesting in the ruins of the PWC building. DOC has built rafts to help the chicks avoid drowning.

Mander said the "extremely adaptable" gulls would likely move to a different site next year. They typically breed on large braided river beds, a dangerous environment which means many chicks do not survive.

The nesting gulls have become a popular attraction in central Christchurch.

The site, formerly the PWC building, is owned by Phillip Carter. He has plans to have deterrents installed once the birds abandon the colony at the end of summer.

STUFF Hundreds of rare black-billed gulls have set up camp at the former PWC building site on Armagh St, in central Christchurch. (Video first published in November 2019)

Carter said he was working closely with DOC to make sure the breeding season was successful.

Plans to build on the site "have been in place for some time" and would be announced in the next month.

The Catholic Church recently spent about $11 million on several blocks of land next to the site, thought to be the first stakes in a larger site for a complex which may include a new cathedral.

Department of Conservation/Supplied The rafts were paddled into position.

The gulls are protected under the Wildlife Act so could not be killed or maliciously disturbed. Anyone who disturbs them or their nests could be fined up to $100,000 and face up to two years in prison.

Black-billed gull populations have declined by 70 to 80 per cent in the last 30 years.