Construction on a contentious $1.25 billion bypass highway in Western Australia's south has been given the green light, after being held up by a lengthy environmental assessment.  

The 27-kilometre Bunbury Outer Ring Road will take traffic around WA's second-biggest city to create a more direct route between Perth and the Margaret River tourist region.

Construction was already well advanced on the northern part of the road, however, the southern link has been tied up in environmental green tape amid strong opposition from the local community.

The highway splices through the semi-rural suburb of Gelorup on a decades-old road reserve.

Local residents have been fighting for the road to be diverted around their community, citing environmental and amenity concerns.

But Main Roads WA has confirmed all environmental approvals have now been achieved, paving the way for the construction of the final leg to go ahead in the coming weeks.

"Federal approval was received at the end of June," a spokesperson said.

The approval by the federal Department of Environment was the last major step in the process before construction on the southern section could begin.

In its consent, the department bound Main Roads to a strict set of environmental conditions to minimise impacts on the local flora and fauna — including the endangered Carnaby's black cockatoo and western ringtail possum.

A department spokesperson described the process as "rigorous".

Locals 'devastated'

Gelorup resident Sue Chapman said she was devastated to learn the project had been considered by the government to be environmentally sound.

"It's just appalling. We are obviously feeling incredibly let down by the supposedly environmental process both at both state and federal level," Dr Chapman said.

Dr Chapman is part of a local lobby group against the road, and said the group would not give up to stop the bypass from proceeding in its current form.

"We've written a letter to [Environment Minister] Tanya Plibersek detailing our concerns about the conditions that have been placed on the construction of this road," she said.

"We feel extremely strongly that Main Roads will not be able to comply with many of these conditions and therefore perhaps the minister should, in fact, not sign off on the road."

Cost blowouts plague project

A total of $852 million was budgeted for the road when it was announced in 2019, however, it was revealed earlier this year that figure had blown out to $1.25 billion.

The increased cost also came with a decrease in the project's scope of work.

Four bridges that would have carried traffic over existing major roads have been abolished from the plans.

It meant a promised 15-minute time-saving on the journey around Bunbury had been abandoned, as had the promise of a free-flowing freeway, with motorists instead having to negotiate roundabouts at key intersections.

The WA government said if it were to build the freeway as originally planned, it would have cost $1.5 billion — almost double the initial cost.

Further cash injections had also not been ruled out, with the WA government's upper house leader Sue Ellery telling parliament in May that additional funding would be sought if it were needed to deliver the project.

Main Roads said construction of the southern portion of the highway through Gelorup would begin in the coming weeks once all relevant environmental conditions had been met.

The agency said the entire project was set to be completed and opened to traffic by late 2024.

A permanent name for the road was currently being considered.