Special tunnels, bridges and fencing are failing to save New South Wales' dwindling koala population from speeding traffic, the state's "road kill" files reveal.

Key points: The files were obtained by the ABC under freedom of information laws

The files were obtained by the ABC under freedom of information laws They reveal dozens of koalas have been killed on the Pacific Highway in the past five years

They reveal dozens of koalas have been killed on the Pacific Highway in the past five years Koala populations are dwindling on the NSW north coast, and are listed as "locally endangered" in several locations there

One tragic incident recorded on video shows an injured koala trapped in supposedly wildlife-proofed tunnel, unable to crawl to safety as traffic roars past.

As tunnel operators try to organise a rescue, a semi-trailer driver ignores flashing warning signs and crosses into the closed lane, killing the koala.

The video is part of the State Government's "road kill" files and was obtained under freedom of information laws from NSW Roads and Maritime Services (RMS).

The files show where koalas are being killed around NSW, and includes five years of incident reports from the Pacific Highway, where 68 koala deaths were recorded.

The video, from 2017, was recorded in the St Helena tunnel, near Byron Bay on the state's north coast, where koalas are listed as "locally endangered".

An incident report about the rescue operation tells how an RMS tunnel operator spotted "something flapping" on a surveillance camera.

After zooming in, the operator focuses on the injured koala.

According to the report, the operator activated LED signs commanding drivers to stay in the left lane, and then made urgent calls for wildlife rescue volunteers to attend.

The report states at around 3:00am: "The tunnel operators turned on UHF radio to Channel 29 and questioned why the truck driver had disobeyed traffic directions and ran over the animal. [The] truck driver brushed it off stating there is no need to rescue the animal now."

After the koala's death the tunnel was closed to traffic, and the rescue volunteers — who arrived soon after — were instructed to retrieve the body.

The ABC understands no sanctions were pressed against the truck driver for breaching either road rules or wildlife laws.

Preventable deaths

It is estimated only about 8,000 koalas remain on NSW's north coast in several colonies.

Dr Steve Philips, one of the world's leading koala experts, and said preventable deaths were pushing smaller colonies "to the precipice of extinction".

"The loss of every one of those animals becomes ecologically very significant," he said.

The St Helena tunnel opened in 2015, and extensive fauna fencing and barriers around the tunnel were designed by wildlife protection experts.

Email reports of the St Helena incident say a "gap in the fauna fencing" allowed the koala to access the tunnel.

The emails also reveal that the next month, a second koala again accessed the highway and was killed very close to the tunnel before repairs were carried out.

A dead koala on the New England Highway, near Armidale, last November. ( Supplied: Harrison Warne )

RMS documents obtained by the ABC repeatedly discuss how even small gaps in existing fencing can create major issues.

On the Pacific Highway, millions of dollars has been spent on animal crossings, land-bridges and hundreds of kilometres of high fences around wildlife corridors.

The road kill files show the Pacific Highway alone claims on average a dozen koalas a year, with the actual toll much higher due to injuries.

An RMS spokesman said it was working to eliminate hotspots on the highway through fencing, koala grids and wildlife bridges.

However, Dr Philips said infrastructure must be backed up by better maintenance.

"When you see maintenance issues being neglected for months on end, and sometimes years on end, you get quite frustrated; because someone's just left a gate open, trees fallen down across the wire and bang — there's a dead animal in a couple of days," he said.

"There is this immense sense of frustration that this iconic animal is being hammered on the roads. And we've got really good knowledge to sort of mitigate impacts where we can."

According to the latest data, koala numbers in NSW have fallen over 25 per cent in two decades — from 45,000 to 36,000

Car strikes far outweigh dog attacks as the cause of accidental death.

On Sydney's south west outskirts nine koalas have been killed on Appin Road, south of Campbelltown, out of a population where as few as 200 animals may survive.

A spokesperson for the NSW Environment Minister said that $3.3 million in funding had been assigned to a program to fix priority hotspots around the state, including upgrades to fencing.