Underneath one of south-west Sydney's busiest roads lies the city's first koala underpass.
At 52 metres long, the Glen Lorne crossing cost $12 million and was designed to reduce vehicle strikes and deaths for one of the state's last disease-free koala populations.
Transport for NSW estimates around 10,000 vehicles use this section of road each day.
But since the underpass opened to wildlife five months ago, not a single koala has been recorded using the safe passage.
That came as no surprise to environmental researcher and policy analyst Colin Salter, who said studies of underpasses in northern NSW and Queensland found koalas rarely used them.
"[These studies] used wildlife cameras within the underpasses themselves as well as outside the structures," said Dr Salter, who also works as WIRES policy lead.
"While there are records of koalas using them, there are not many, and they are infrequent … like one or two detections.
"But the research has also said that there is not enough data, even though there are repeated results from different studies that show there's a low use by koalas."
In 2022, researchers from the University of Southern Queensland spotted only one koala and a handful of native animals using crossings under the Toowoomba bypass throughout a 12-month study using camera traps.
This was similar to Southern Cross University's 2021 study, which detected close to 5,000 medium-to-large mammals and goannas using highway underpasses at Port Macquarie and Grafton.
Dr Salter said studies showed it could take a "significant period of time" for wildlife to start using new crossing structures.
Overpasses preferred
Dr Salter said overpasses were a more effective way to protect animals from vehicle strikes.
"There appears to be a preference for overpasses as they are wildlife crossing structures that replicate habitat," he said.
"There's research in Australia, but also internationally, which shows this."
The NSW government told the ABC it knocked back an early proposal from WIRES to build an overpass at Glen Lorne as additional land clearing would have been required.
Dr Salter said although it was a disappointing decision, the underpass could help fill important knowledge gaps.
"It provides us with a significant opportunity to monitor the movement of animals on either side of the underpass in the adjacent habitat, but also if they are moving towards and through the underpass."
The box-culvert structure was delivered by developer Stockland, which is building the neighbouring 1,700-home Figtree Hill Estate.
Construction of the crossing was held up while Stockland sought access to Lendlease-owned land on the opposite side of the road.
The Sydney Basin Koala Network (SBKN) said BioNet data showed there had been six koala strikes and five deaths during the delay period, which Stockland described as "regrettable".
SBKN said 66 koalas have died on Appin Road since July 2022.
More Appin Road underpasses
Stockland is now due to build another Appin Road underpass near the Beulah Nature Reserve.
NSW Environment Minister Penny Sharpe said she wanted to avoid the delays that left the Glen Lorne crossing almost a year behind schedule.
"Sometimes delays happen. I still think this is absolutely worth it," she said.
"Hopefully the lessons that we have learnt here will see works sped up.
"I understand why people are concerned about it as it is a precious koala population."
The developer has told the ABC it has secured a land access licence with Lendlease to facilitate Beulah's delivery and that environmental applications would be lodged in the coming months.
The final Appin Road underpass, Ousedale, is a Transport for NSW project slated for the end of 2027.
