The West Australian Government has won an appeal against a Supreme Court decision which ruled environmental approvals for Perth's controversial Roe 8 highway extension were invalid.

Key points: EPA policies "not mandatory considerations" for Roe 8, court says

EPA policies "not mandatory considerations" for Roe 8, court says State Government welcomes decision

State Government welcomes decision Opponents considering High Court appeal

Supreme Court Chief Justice Wayne Martin ruled last December 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.

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

However in a unanimous decision, the Court of Appeal has now ruled in favour of the State Government, finding the EPA was not obliged to take its own policies into account.

"The policies are not mandatory relevant considerations," Justice Carmel McClure found.

The court found the policies were "a permissive relevant consideration, not a mandatory relevant consideration".

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.

The Save Beeliar Wetlands group said it hoped to continue its legal fight against the project despite the Court of Appeal's ruling.

"It is disappointing, however it's not the end," Save Beeliar Wetlands convenor Kate Kelly said.

"We can seek special leave to challenge this decision in the High Court and we will certainly be pursuing whether that's possible."

Protesters outside the court vowed to continue their fight against Roe 8. ( ABC News: Andrea Mayes )

Government vows to push on with Roe 8

Mr Jacob welcomed the court's decision and said it vindicated the Government's beliefs about assessment and approval processes.

"Policies are not legislation, therefore policies are there as guidelines, not to be mandatory relevant considerations," he said.

Albert Jacob says the ruling means the Government's Roe 8 approvals are valid. ( ABC News: Andrew O'Connor )

Mr Jacob said the Government would be pushing ahead with Roe 8.

"We will be acting as if they are valid approvals ... there's a question of whether the High Court would take this matter or not so it's simply to early to say, but the legal position for those environmental approvals for Roe 8 is that they stand," he said.

However Shadow Environment Minister Chris Tallentire also said the ruling was disappointing.

"The decision that Chief Justice Wayne Martin put out late last year was very clear, that there was an offsets policy in place that the EPA hadn't even referenced in their review of the whole proposal," he said.

"[Roe 8 will] see the destruction of our Beeliar Wetland, see the destruction of communities, severing a community with a black road four lanes wide going through the middle of it."

Thousands of protesters have attended anti-Roe 8 rallies, with environmentalists arguing clearing the Beeliar Wetlands would threaten wildlife habitats.

Drilling was also disrupted by activists when it began last December.

Government should 'back off': Opposition

Labor has vowed to axe the Perth Freight Link if it wins next year's state election, and again warned the Government not to lock in Roe 8 contracts prior to the March poll.

It has previously cited a report by the Victorian auditor-general, which found the abandoned East-West Connect project had cost Victorian taxpayers a total of $1.1 billion.

"If the current Government goes ahead and signs expensive contracts that's going to put the state into further debt," Mr Tallentire said.

"It's time they realise this project has no community support and they back off and they let the people decide in March 2017."

But Mr Jacob was standing firm.

"Any decision on the contract and when the project starts are really for the Premier and the Minister for Transport," he said.

The Supreme Court's decision last year resulted in an independent review of the EPA recommending a radical overhaul of its policy framework.

The review, initiated by Mr Jacob and led by Peter Quinlan SC, found the agency needed to be massively simplified and made clearer.

The Government had also started a fresh Roe 8 environmental assessment process independent of the EPA.