WA Labor will axe the $450 million extension of Roe Highway across the Beeliar Wetlands and the entire Perth Freight Link if it wins the state election in March.

Key points: Labor says scrapping the Perth Freight Link will free up $1.7 billion

Labor says scrapping the Perth Freight Link will free up $1.7 billion It has promised a new port at Kwinana and three new road projects

It has promised a new port at Kwinana and three new road projects But the Government says the decision is "reckless and costly"

Opposition Leader Mark McGowan said the Roe 8 extension contract would be renegotiated, freeing up a total of $1.7 billion for other projects.

The State Government released the contract in October, with Premier Colin Barnett predicting Labor would complete Roe 8 if elected.

But Mr McGowan today confirmed Labor would halt construction of the road, an essential element of the Barnett Government's $1.9 billion Perth Freight Link project.

He said Labor would reallocate $236 million to three new projects, including bringing forward the Armadale Road dual carriageway project.

He has also pledged to build the Armadale Road Bridge, linking North Lake and Armadale roads, over Kwinana Freeway, to better connect suburbs in Perth's south.

Mr McGowan said the Opposition had independent legal advice, from former WA solicitor-general Grant Donaldson, who indicated the projects could be stopped at a modest cost.

"Today we are righting a wrong - Colin Barnett should never have signed contracts so close to the election," Mr McGowan said.

"Today's decision is gutsy ... but it's in the interests of taxpayers and the people of Western Australia."

He said Labor would immediately start work on planning a new port at Kwinana to complement Fremantle Port, which is nearing capacity.

Mr McGowan said he expected the Commonwealth to support any decision to scrap the Roe 8 project, just as they supported the Victorian Labor Government's decision to scrap the East-West project.

The Federal Government committed to jointly fund the Perth Freight Link project in May 2014.

"We would expect the same treatment as Victoria received and I would not take any notice of the blusterings of ministers, federal ministers, before the state election," Mr McGowan said.

However, a spokesman for Urban Infrastructure Minister Paul Fletcher said the $1.2 billion committed by the Federal Government was "specific to the project" and "cannot simply be re-allocated" to other projects.

'A silly idea': Transport Minister

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Transport Minister Bill Marmion described Labor's decision as "reckless and costly".

"I think that is a silly idea. Roe 8 is a key component of decongesting the southern suburbs," he said.

He said the cost of breaking the contract would be significant.

"It could cost taxpayers of Western Australia $30 [million] or $40 million if the Labor Government decided not to build Roe 8. Reckless is probably understating how bad that decision is."

The Roe 8 project would extend the highway five kilometres as a dual carriageway road, from its current Kwinana Freeway end in Jandakot to Stock Road in Coolbelup, through the Beeliar Wetlands.

It is the first stage of the Perth Freight Link, a heavy haulage route designed to get freight to Fremantle Port.

Roe 8 has attracted ongoing protests from environment and Indigenous groups. ( ABC News: Andrea Mayes )

The Government says building Roe 8 will take 2,000 trucks off Leach Highway between the freeway and Stirling Highway each day, and thousands more off other surrounding roads.

Roe 8 has attracted major support from the transport industry while also drawing heavy criticism from a coalition of environmentalists, conservationists, Indigenous groups and residents.

Just last month, the Save Beeliar Wetlands group lost its last avenue of appeal against the project, when the High Court refused it special leave to appeal against a Court of Appeal decision which revalidated environmental approvals.

The conservation group is pursuing alternative legal action in the Federal Court to challenge federal environmental approvals.