The controversial Roe 8 highway extension in Perth has been dealt a major blow after environmental approvals of the project were ruled invalid by the Supreme Court.

The Save Beeliar Wetlands group had brought the case to court, arguing the Environmental Protection Authority did not take its own policies concerning land offsets into account when it assessed the project.

The legal bid succeeded today when Chief Justice Wayne Martin ruled the EPA's assessment and recommendation to Environment Minister Albert Jacob was invalid, along with his subsequent approval.

Roe 8 is a key part of the $1.6 billion Perth Freight Link project, which seeks to build a byway for heavy haulage from Perth's eastern industrial suburbs to Fremantle Port.

Thousands have attended anti-Roe 8 rallies in recent months and drilling was disrupted by protesters when it began last month, over fears it would destroy parts of the Beeliar Wetlands in Perth's south.

In his judgment, the Chief Justice said the EPA took no account of its published policies when it recommended to the State Government the project be approved.

He found the EPA was legally obliged to follow its own policies.

"The court concluded that the EPA took no account of its own published policies at the time it made its decision and provided its report to the Minister," Chief Justice Martin wrote in his judgment.

"The court therefore concluded that the environmental impact assessment undertaken by the EPA was invalid."

Cheers as judgment announced

Members of the wetlands group had packed into the public gallery for the decision, and there were cheers as the Chief Justice announced his judgment.

Outside the court, group members said it was a historic day but added that the fight to stop the project was not yet over.

Save Beeliar Wetlands convenor Kate Kelly said she was overwhelmed by the court's ruling.

"It's really a vindication of our efforts over so many years," she said.

"What it does say is that the EPA didn't follow its own guidelines in making this judgment and that's very significant.

"It's a very encouraging move in terms of actually hopefully changing things for the better."

Greens senator Scott Ludlam paid tribute to the "extraordinary" community campaign, volunteers and the legal team after the decision was handed down.

"The slogan that this extraordinary movement has adopted in the last couple of months has been 'Rethink the Link', this is the opportunity to rethink the link, hopefully this is the circuit breaker," he said.

Loading

WA Greens MP Lynn McLaren said the case should not have needed to come to court in the first place.

"The Barnett Government needs to pay attention to its environmental laws," she said.

"Clearly the EPA has failed yet again this state, and thank goodness we had a community group, that was supported by public funding that would challenge that decision and a court of law that would quash it.

"This shouldn't be the case, the environment is protected by law in Western Australia and those laws should be upheld and improved not ignored."

Roe 8 will be 'significantly delayed': Barnett

Premier Colin Barnett said he was disappointed with the Chief Justice's ruling but respected his decision.

"We've yet to have time to really study the full implications of that but my best guess at this stage is it will mean the State Government will need to re-do all or part of the environmental approvals for Roe 8," Mr Barnett said.

The five-kilometre Roe 8 extension is designed to take Roe Highway from Kwinana Freeway to Stock Road. ( ABC News: Andrew O'Connor )

"If that's the case we will proceed on that basis and we will go ahead with Roe 8."

But Mr Barnett said the ruling would result in "a significant delay" to the project.

"It means that the hundreds of jobs will be delayed and indeed the danger in terms of traffic in the area will continue longer than I would have hoped," he said.

"But the State Government is committed to Roe 8 so if we have to re-do it, that's what we will do, and the project will proceed."

Mr Barnett said he understood the ruling related to the EPA's use of environmental offsets, setting aside additional land for preservation to compensate for the loss of bushland.

"I know lots of people are celebrating today thinking they've won - well, I'm not going to get into a fight with the community, but as I see the ruling, the actual project is okay. It's about offsets," he said.

A spokesperson for the EPA said the board would take time to fully consider the findings.

"For this reason, it would be inappropriate to comment further at this stage," the spokesperson said in a statement.

Mr Jacob said in a statement he would consider the judgment and take advice before deciding what to do next.

Project should be abandoned: Opposition

Labor spokesman Chris Tallentire said the court's ruling meant the project should be scrapped.

"The fact that the EPA were not able to properly consider the environmental protection policy, an offsets policy that was in place at the time, just shows how flawed the assessment was," he said.

"The federal environmental assessment was contingent on there being a valid state assessment in place, so this is great news for the protection of the wetlands.

"This project is just not going to go ahead."

However City of Melville Mayor Russell Aubrey said he was disappointed by the court's ruling and the potential delays to Roe 8.

"It's not just about freight, it's about traffic, it's about the impact on those intersections, it's about the impact of the mixture of freight and light vehicles and the trauma that's causing on roads such as South Street and Leach Highway," he said.