The Western Australian Government has defended its decision to withhold documents on the Perth Freight Link project from the Senate.

The Senate had sought to table the Government's business case for the $1.6 billion road project, which has come up against strong opposition from residents and environmental groups.

The Barnett Government has refused to hand them over, leading to accusations from the State Opposition that it has something to hide.

But Transport Minister Dean Nalder said the Government was protecting commercial-in-confidence information and was simply following proper process.

"The response that we're providing is no different than the response that Alannah MacTiernan provided when being questioned about the Mandurah rail line," he said.

"When it comes to commercial-in-confidence information, and we are in the process of contract negotiations, we don't release that."

The Senate yesterday passed a motion moved by WA Greens senator Scott Ludlam, supported by federal Labor, demanding the Barnett Government release its traffic modelling, cost benefit analysis and business case.

Senator Ludlam said with the proposed route passing through a wetland and residential suburbs, the information should be in the public domain.

The Senate sought the information from Infrastructure Australia, which had considered WA's funding request for the project.

Infrastructure Australia, in turn, sought advice from the director-general of WA's Department of Transport (DoT), who indicated his department had a significant objection to releasing the documents.

Key information already available, transport chief says

DoT director General Reece Waldock cited commercial in confidence and the potential impact on state-federal relations as reasons for opposing the release.

In a formal response to Infrastructure Australia, Mr Waldock said key information was already available.

"A 30-page business case summary of the project has been publically available on Main Roads website since the project was announced in late 2014," he told Infrastructure Australia.

Opposition Transport spokeswoman Rita Saffioti says the public deserves to see the documents.

"This summary includes a breakdown of the benefit-cost analysis results, details of the problems the project will resolve, the current challenges facing Western Australia and the freight system and the scope of the project."

In Parliament, Mr Nalder said the Government would disclose the information when the tender process for the project was complete.

"We will be more than happy to provide the information when it's no longer commercially in confidence or we've completed the contractual negotiations," he said.

The Opposition's transport spokesperson, Rita Saffioti, said releasing the documents now would help answer some of the questions about the project's costs and benefits.

"What has this Government got to hide? Sure, that can justify what they believe is committed expenditure?" Ms Saffioti said, speaking outside Parliament.

"They're hiding, yet again. They are hiding the key documents that the WA public and the Australian public deserve to see."