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)

The most threatened gull on Earth has set up a colony in a derelict Christchurch city centre site.

About 300 black-billed gulls, tarāpuka, have established a colony with about 130 nests in the half demolished and flooded foundations of a former office block on Armagh St.

The birds, which can only be found in New Zealand, are nesting on the concrete beams, mating and washing in a flooded basement surrounded by protective fences.

JOSEPH JOHNSON/STUFF Black-billed gulls usually nest on the braided rivers of the South Island.

Department of Conservation (DOC) chief science advisor Ken Hughey said it was "extremely rare" for the gulls to set up a colony in a city.

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They usually breed on islands in the braided rivers of the South Island.

JOSEPH JOHNSON/STUFF The gulls are often seen on the coast and in estuaries during the winter.

"It is the first one I have heard of," he said.

"That is quite a secure spot for them. It has a fence around it so they can nest where predators can't get them."

Hughey said the birds do not necessarily return to the same site to breed each year, although they have nested on Armagh St for the last two breeding seasons, which run from September to February.

JOSEPH JOHNSON/STUFF The birds are categorised as "nationally critical'' and are protected.

The gulls are protected under the Wildlife Act so could not be killed or maliciously disturbed.

Anyone who disturbs the gulls or their nests could be fined up to $100,000 and face up to two years in prison.

DOC senior biodiversity ranger Anita Spencer said they were monitoring the colony.

STACY SQUIRES/STUFF The flooded foundations of the former PWC office block on Armagh St have been sitting derelict for many years.

"DOC has been working very positively with the landowner, the Carter Group, on sustainable options to help this colony have a successful breeding season, which should boost the population locally.

"We ask that if people see groups of tarāpuka in the city centre, to enjoy their presence, but give them a little space and leave them be."

Site owner Philip Carter said he was "delighted that the tarāpuka have made their home in the city".

JOSEPH JOHNSON/STUFF The breeding season for the gull lasts from September to February.

A DOC spokesman said at the end of the breeding season, once the colony has been abandoned by the gulls, the owner would legally be allowed to "put measures up to make the site less attractive to birds".

​Academic Elizabeth McKenzie, who was recently elected as an Environment Canterbury councillor, has set up an online petition calling for the colony to be protected until the bird is no longer endangered or the colony has been abandoned.

An email about the colony from Christchurch City Council ornithologist Andrew Crossland was shared on Facebook by one of the recipients.

JOSEPH JOHNSON/STUFF The threatened birds are happily mating in the city centre colony.

Crossland said the bird was "a quintessential Canterbury species" and "an iconic resident of braided river habitats".

"The grated 'windows' along the fence [around the site] offer a fascinating insight into gull breeding ecology 101. As good as a bird hide at any wilderness nature reserve," he wrote.