An environmental group has lost its last avenue of appeal against the Western Australian Government's plans to build a highway extension, which goes through a wetlands area in the Perth suburb of Beeliar.

Key points: Court concludes "insufficient grounds" to appeal

Court concludes "insufficient grounds" to appeal Roe 8 highway to extend to Fremantle Port through wetlands

Roe 8 highway to extend to Fremantle Port through wetlands WA Court of appeal had previously ruled EPA approvals invalid

The High Court has today refused the Save Beeliar Wetlands Group special leave to appeal against a Court of Appeal decision, which revalidated environmental approvals for the controversial Roe 8 highway extension.

The court concluded there were "insufficient grounds" to appeal the Court of Appeal decision.

Kate Kelly from Save Beeliar Wetlands said the High Court decision was a blow to the group's efforts to halt the Roe 8 project.

"There's no getting around it. It is a disappointing result," she said.

Lawyers for Save Beeliar Wetlands argued the Court of Appeal had not correctly considered that the EPA had been legally unreasonable in ignoring its own policies when approving the project.

Lawyers for the Government rejected that argument, and told the hearing the EPA was under no legal obligation to consider its policies, and was therefore free to grant approvals on other considerations.

After submissions lasting 20 minutes each, the High Court ruled there were "insufficient grounds" for an appeal against the Court of Appeal decision.

The High Court decision exhausts Save Beeliar Wetlands legal options in this case, but Ms Kelly told reporters the group was pursuing other legal avenues.

"We do have another legal option available to us. We do have a legal matter on foot in the Federal Court," she said.

"So there is still a cloud of unresolved legal capacity over this project."

But not in the mind of Premier Colin Barnett, who welcomed the result.

"I'm glad all that's now finally exhausted and we can simply get on, build the road and take up to 8,000 heavy vehicles, trucks off suburban roads and put them on a dedicated Roe 8," he said.

Kate Kelly urged the Government to let the public decide the project's future at the March state election.

Kate Kelly from Save the Beeliar Wetlands group says there are still legal avenues open to them. ( ABC News: Andrew O'Connor )

Mr Barnett said construction had already been delayed by year and would now push ahead.

"They've made their point. They've taken every legal avenue open to them. I've respected that. I'm frustrated that its delayed the project for 12 months. Now construction will just proceed," he said.

Roe 8 is a key part of the $1.9 billion Perth Freight Link project, which seeks to build a byway for heavy haulage from Perth's eastern industrial suburbs to Fremantle Port.

It has been a flashpoint for the Barnett Government since the Commonwealth committed to jointly fund the Perth Freight Link project in May 2014.

The project aligned then-prime minister Tony Abbott's desire to fund major road infrastructure projects with the Liberals' long-held intention to build Roe 8, the first stage of the Perth Freight Link, by extending Roe Highway to Stock Road and then constructing Roe 8 to Fremantle.

But it immediately provoked strong opposition on two fronts.

Environmentalists condemn highway plans

Environmentalists, conservationists and Indigenous groups condemned the plan to extend Roe Highway across part of the Beeliar wetlands.

And residents along a section of Leach Highway and business owners and residents on Stock Road faced the forced sale of their properties to make way for the new roads.

As protests gathered momentum, the Save the Beeliar Wetlands group mounted a legal challenge, arguing that in approving the project, WA's Environmental Protection Agency had ignored its own policies.

In December 2015, Supreme Court Chief Justice Wayne Martin ruled that the Environmental Protection Authority's (EPA) assessment of the project and recommendation to Environment Minister Albert Jacob was invalid, along with his subsequent approval.