The Barnett Government spent $80,000 to get advice on how to procure the MAX light rail project just weeks before deferring a decision on it until at least 2017.
Treasurer and Transport Minister Troy Buswell has refused to release the "procurement options assessment" report by accounting giant PricewaterhouseCoopers or a summary of its recommendations.
Tender documents show the $80,000, one-month contract for an options assessment was awarded on November 5.
On December 18, Mr Buswell revealed a decision on MAX and its 22km of light rail from Mirrabooka to the city and the University of WA had been shelved until beyond the next State election.
The project, which the Government promised during last year's election campaign would be running by 2018, was delayed to 2019 in the State Budget. Now, it says it will not make a decision until June 2017, meaning the project will not be finished before 2022.
Before deferring it, Mr Buswell repeatedly said the project would likely attract interest from the private sector.
Mr Buswell's office would not discuss options the report looked at nor how much it influenced the deferral. A spokeswoman for Mr Buswell said the report would not be released because it was subject to Cabinet confidentiality.
She said the MAX team would continue to work on the project's business case, which was crucial to settling its requirements and would allow for a "procurement decision" in three years.
Shadow transport minster Ken Travers said the Government's refusal to release the report or answer questions "can only lead you to speculate it was going to blow out further in cost".
"That could be another reason they've delayed it," he said.
"They've constantly argued they would get private financing but the reality is that was never going to reduce the cost."
He believed the Government wanted savings from a public-private partnership that probably were not there to be had.