A large colony of black-billed gulls on the Ashley River. (Video first published November 2018)

A driver has run over more than a dozen of the most threatened gull species in the world's fledgings in North Canterbury.

Ashley Rakahuri Rivercare Group volunteer Grant Davey discovered the carnage on Thursday when monitoring a colony of about 800 birds on the Ashley riverbed.

A four-wheel-drive vehicle was thought to have run over the young birds on Wednesday night, marring a potentially excellent breeding season. Tracks indicated the vehicle had driven on the edge of the colony.

Department of Conservation More than 12 black-billed gulls killed on the Ashley River in Canterbury.

In November, the arrival of the largest colony of black-billed gulls seen on the river in almost 20 years prompted calls for users to take extreme care. Signs were erected to highlight the importance of breeding birds.

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Black-billed gulls are endemic to New Zealand and are the most threatened gull species in the world. The Department of Conservation estimates there are about 60,000 breeding adults left, and the population is expected to decline by up to 70 per cent over 10 years – or three generations.

Department of Conservation One of the black-billed gulls believed to have been run over by a four-wheel-drive vehicle in Canterbury.

Rivercare Group chairman Nick Ledgard said there was "no excuse for anyone to be driving through the bird colonies".

He said they had an "excellent proactive relationship" with the Combined 4WD Club, which did an "excellent job" in encouraging members to stay away from critical breeding sites from October to the end of January.

Black-billed gulls, which are found mainly around braided rivers in the South Island, have been declining in numbers since about 1980. They are under threat from habitat loss, introduced predators, human disturbance and the larger black-backed gull.

JORDAN KAPELLY/STUFF Black-billed gulls are the most threatened gull species in the world.

People disturbing, distressing and killing the black-billed gulls were committing an offence under the Wildlife Act and could be prosecuted, Department of Conservation senior ranger Sandy Yong said.

It is an offence under the Wildlife Act to harm or kill protected wildlife and if prosecuted, offenders may be liable for a fine up to $100,000 or up to 12 months imprisonment.

Anyone seeing vehicles disturbing breeding sites on the riverbed is to take note of their details, photograph if possible, and contact the local DOC Rangiora office, or call the DOC Hotline on 0800 362468 if after hours.