A leaked federal government advisory board report that does not make a case for the contentious Perth Freight Link is a major embarrassment to the Liberals, the opposition says.
The leaked report by Regional Development Australia instead describes an outer harbour further south at Kwinana as a "nation building" project that requires strong government intervention.
The federal government has committed to funding almost 63 per cent of the three-stage Perth Freight Link, with the first two phases estimated at $1.9 billion.
Labor says there is no logic in pushing ahead with the link as it will lead to the century-old Fremantle Port, which the report says will reach full capacity between 2025 and 2035.
The state government estimated last year that construction would be complete by 2019, but the time frame has slipped because of blunders in the environmental approvals process.
Federal Labor MP Alannah MacTiernan says pursuing the unpopular project is "completely irresponsible".
"What this report is telling us is very clear: that the number one priority in Western Australia, in Perth, is development of the outer harbour," Ms MacTiernan told reporters on Sunday.
"If we don't proceed with developing the outer harbour and making sure that we have got efficient port infrastructure ... this will damage our economy into the long term. This will be selling WA short.
"It is outrageous. This is not the way to go."
She said both major parties had been working on an outer harbour plan for the past 20 years, but the link had been "dreamt up" by WA Treasurer Mike Nahan and federal Finance Minister Mathias Cormann without any consultation.
"Since then, they have been scrambling to develop a business case to justify it."
The first stage of the link, known as Roe 8, has conservationists up in arms as it will cut through the Beeliar wetlands, a home to the endangered Carnaby's black cockatoo.
The second stage is a concern for residents in the path of a tunnel as it will require properties to be compulsorily acquired and bulldozed.
The third stage has not yet been decided upon, prompting opponents to label the link a "road to nowhere".