The West Australian Opposition has seized on a report from Infrastructure Australia which it says shows the Perth Freight Link project has been hastily conceived and poorly planned.

The assessment was conducted by Infrastructure Australia in May and has just been released.

The Opposition said it showed the Perth Freight Link was not part of long-term planning, and did not adequately consider other options, including the construction of a new outer harbour in Cockburn Sound.

"We believe this is a flawed project. We don't believe the work has been done to justify the $2 billion expenditure," Opposition transport spokeswoman Rita Saffioti said.

"They still don't know the route. They have no idea how they're doing to get this project done."

She pointed to Infrastructure Australia's assessment of the freight link's alignment with state priorities.

The report said the WA Government's business case "outlines strong links" with state priorities and policies.

But "at the time of the assignment (May 2015), the Perth Freight Link is not directly mentioned" in eight key strategic planning documents, the report found.

They included the State Planning Strategy 2050, Direction 2031 and Beyond, the WA Regional Freight Transport Network Plan, Draft Perth Freight Transport Network Plan, the Draft State Port Strategy Plan and the Fremantle Port Inner Harbour Port Development Plan.

Ms Saffioti said it was clear that the project was given priority for political, rather than strategic reasons.

"This is a project that the Abbott Government wanted to fund because they didn't want to fund public transport," she said.

"Now they [the State Government] have been forced to this project and what we're seeing is Colin Barnett going ahead with this project basically because the Abbott Government has given them some money."

Alternatives not properly considered: report

The Opposition also questioned the selection and assessment of the Perth Freight Link over alternative projects.

The Infrastructure Australia report showed the freight link was selected from a shortlist of four proposals, culled from a list of 12 options.

The report showed the option of an outer harbour was never considered, and that of the 12 options, only the Perth Freight Link was subjected to any detailed assessment.

"A rapid BCR (Benefit Cost Ratio) was not completed for additional options to determine if the preferred option provided the greatest net benefits," it said.

Infrastructure Australia's assessment concluded while there was a lower cost option, it did not generate the $2.4 billion benefit predicted to flow from the freight link project.

The report questioned some of the modelling used to justify the project, but concluded that it would deliver substantial economic benefits.

In Parliament, WA Treasurer Mike Nahan rejected the Opposition's criticism, and said long-term and detailed work had been done.

He said following the election of the Liberal-National Government in 2008, the Government spent $20 million planning construction of Roe 8.

"We did the hard yards - community consultation, planning the roads, getting the optimal route, going through environmental clearance," he said.

"We did it all, and it's ready to go."

He said the project was only delayed when the then federal Labor government refused to fund the project.

Dr Nahan also rejected the Opposition's criticism of building a freight link to Fremantle when the port would reach capacity, forcing construction of an outer harbour.

He said Roe 8 would be needed in either case.

"Roe 8 is essential for both the inner and outer harbour," he said.

"You get to Stock Road, you turn right and you go to the inner harbour [Fremantle], eventually you'll need to turn left and so south and go into the outer harbour, when in 10 to 15, maybe 20 years it's being built," he said.