Preliminary work has begun on the first stage of the controversial Perth Freight Link, despite the possibility of a High Court challenge proceeding.
Protesters and police stood by as fencing was put up on Monday, with substantive work expected to begin later this week.
The proposed Roe 8 highway extension cuts through the Beeliar Wetlands in Perth's south, and activists have opposed the project as they say it will destroy the area, which is a habitat for the endangered Carnaby's black cockatoo.
Protesters are now also particularly concerned about the impact on Rainbow Bee-eaters, protected migratory birds who use the Beeliar Wetlands to nest until the end of February.
Greens MLC Lynn MacClaren said it was economically and socially irresponsible for work to begin despite the looming legal challenge.
Save Beeliar Wetlands activists will head to the High Court on December 16 for a special leave hearing, where it will be decided if a full hearing will proceed.
Convener Kate Kelly said growing community and local government opposition to the project showed there were unresolved issues.
"The move to start works before legal matters are resolved is provocative and will damage wildlife and this beautiful bushland and wetland," she said.
Rethink the Link co-ordinator Kim Dravnieks said Rainbow Bee-eaters had been missed in the environmental management plan for the project.
Ms Dravnieks said securing High Court leave was difficult but the group remained hopeful.
"If we're unsuccessful, there are other legal avenues we'll be going through. We are certainly prepared to keep going," she told 6PR radio.
WA Labor leader Mark McGowan said while his party did not support the project, they would wait until after the High Court decision to announce whether they would tear up contracts if they win the March state election.
He criticised the government for warning protesters to not break the law, while beginning work before clearing all legal hurdles.
"In my view, the government itself is now flouting the legal process and it's a bit rich of them to criticise people for protesting," he said.