Contracts for the controversial Roe 8 project will be signed this week, West Australian Premier Colin Barnett has announced.

The project, part of the Perth Freight Link extension which aims to connect the city's industrial east to the Fremantle Port, has attracted controversy after the state's environmental watchdog was found to have failed to follow its own protocols.

But an appeal overturned this decision, which environmental activists have vowed to take to the High Court, leaving a legal cloud hanging over the project.

The Government will spend $340 million extending the Roe Highway between Kwinana Freeway and Coolbellup Avenue, and $110 million building a Stock Road interchange.

Of this, $45 million will be spent on environmental conditions and regulations.

The total Roe 8 project budget was projected to be $540 million. The Government now says it will spend $450 million, $90 million under budget.

The contract will be awarded to a consortium comprising CPB Contractors, Georgiou, WA Limestone, GHD, AECOM and BG&E this week, the Premier said.

The five-kilometre four-lane extension of the Roe Highway is aimed at diverting heavy haulage trucks off existing routes through the city.

The construction is expected to take three years, and the Premier said it would create hundreds of jobs.

The overall Perth Freight Link has been projected to cost $1.9 billion, and is set to be funded by both the State and Federal governments.

However, the project has attracted widespread protest over fears it would negatively impact the Beeliar Wetlands.

A legal bid to stop it based on Aboriginal heritage concerns was thrown out by the Supreme Court in August.

The State Government last month announced a variation to the contract meet an unspecified additional cost to connect the project to Stock Road.

Congestion relief for motorists: Premier

Mr Barnett said the project would deliver relief for motorists, taking 7,000 trucks off suburban roads.

But it has been labelled a "road to nowhere" by its critics, who argue is simply shifts congestion from Leach Highway to Stock Road.

The State Government has swept aside the risk of a potential High Court challenge being launched by environmentalists.

Protests were held when drilling began at Beeliar Wetlands. ( ABC News: Jessica Strutt )

"Our advice is that is unlikely to be accepted by the High Court and even if the High Court does accept it, it is very unlikely to succeed," he said.

WA Labor opposes the project and has urged the Government not to sign contracts ahead of the state election.

Mr Marmion said there would be no provision in the contract to halt the project with either a successful court challenge or a change of government.

"If another government in the future wants to stop the progress, that's their call. We're a Liberal-National Government. We're building Roe 8."

The Government has already faced substantial protests from environmental, conservationist and Indigenous groups, who have also threatened to disrupt construction work.

Mr Marmion said he was not expecting trouble, and asked by a reporter how many police would be needed to build the road, he offered a one-word response.

"None," he said.