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The Ashley-Rakahuri Rivercare Group works tirelessly to maintain the river and the wildlife that inhabit it.
Member Grant Davey said this year it has had the second biggest black-billed gull colony on the river in 20 years.
"We try to look after the birds that nest here. People see a group of the gulls and think 'oh they're just seagulls' but they're not.
"The black-billed gull is currently listed as nationally critically endangered."
That means they are in the most severely threatened group, facing an immediate high risk of extinction.
But Davey said in recent months there has been about six separate incidents of motorbikes or four-wheeled drive vehicles driving into the birds' nests.
"Two motorbikes came racing through a group of young gulls and later I found a number of dead birds near that area. We can't prove it was the motorbikes but I think it was."
The group has motion-detecting trail cameras set-up at the river.
"Another incident recently we caught on camera was a four-wheeled drive which was driving straight up to the gulls and they all took off in a big cloud and the vehicle just kept going straight through it.
"There are still a number of young birds that can't fly yet."
"A lot has been done. We've put signs up, tried to educate people. Environment Canterbury have put fences and gates up. It worked for a while but recently we've seen a spike in the incidents."
The issue has got worse recently due to the low river levels, meaning people can race through them, Davey said.
Dotterels and other native birds live on the river. Their nesting season usually runs from about July to February each year.
Under the Wildlife Act, penalties for hunting, killing or disturbing absolutely protected wildlife can be up to five years imprisonment or a $300,000 fine.
"It is just reckless and absolutely unacceptable behaviour."