Cawleys Bridge retrofit reconnects wildlife habitat above Princes Motorway

High above the traffic, a concrete bridge is being transformed into a thoroughfare designed not for cars, but for wildlife.

An estimated 40,000 vehicles a day pass beneath Cawleys Bridge, unaware it could one day help reconnect ecosystems split apart by the road below.

The M1 Princes Motorway, near the Illawarra-Heathcote corridor, is one of the state's busiest commuter routes and an almost impossible barrier for animals trying to move between habitats.

Now the long-overlooked structure is being turned into a living corridor, allowing animals to safely cross from one side of the landscape to the other.

For retired National Parks ranger Bob Crombie, it is the realisation of an idea he first had as a young man in the mid-1970s.

"I came up with this idea in about 1974, 1975, I was working in the park as a ranger naturalist … and it became apparent that [wildlife] needed to be able to move around the landscape," he said.

"The movement of the species that keep those places viable has been blocked by this great big highway."

Mr Crombie said back then government departments were only beginning to understand how essential movement was to the survival of native species.

"This park [Royal National Park], everyone thinks is 14,000 hectares," he said.

"It's actually a number of small units, so it is really quite threatened when you think of those units."

Looking at maps, Mr Crombie saw the potential of the forgotten crossing.

With the support of various government departments, that corridor is now being built, not by constructing something new but by transforming what was already there.

Kylie Madden, an ecologist working on the project, said the retrofit approach was what made it significant.

"So this is probably one of the most exciting projects I've ever been involved in, in my entire career," she said.

"It's a retrofit of an existing bridge."

Originally, the structure existed simply to allow occasional service and emergency access.

Now the bridge deck itself is being converted into functioning habitat, although it will still be able to accommodate vehicles.

More than just a crossing

Logs recovered from recent land clearing at the Mt Ousley interchange have been used to create pathways.

Soil and vegetation will soon be installed to mimic the forest floor.

It has been designed to create features that encourage animals to cross as if they are still moving through continuous bush.

"Every bit of the structure has been thought through very carefully," Ms Madden said.

Elevated structures and ropes will allow gliders and possums to travel above ground, while ground cover will offer smaller animals protection from predators.

"There were about four or five different butterflies actually crossing in there already," Mr Crombie said. 

Transport for NSW director of regional delivery and statewide services Matthew Burns said it was a "really great opportunity for our bridge carpenters to work on large timbers and get that experience and exposure to these unique projects". 

"It is great to see how the various different parts needed to come together and get the best outcome, and see from a koala's point of view what makes it attractive ... and how they get from one side of the motorway to the other," he said.

The first tiny footsteps

Cameras are already in place and will be monitored for several years to track which species return and how they use the crossing. 

"This is the first retrofit of an existing structure [for use as a wildlife corridor] that we know of in Australia," Ms Madden said.

Mr Crombie said seeing it built after half a century was deeply personal.

"Look, it feels terrific. I didn't think I would see it," he said.

"What's being done here is wonderful thinking … recognition of wildlife is important.

"This is not one, this is only the beginning."