Treasurer Troy Buswell has challenged the Labor Party to submit its Metronet rail plan for Treasury analysis after claiming he had uncovered a $860 million black hole in the airport rail link.
After three successive days of Metronet announcements by Labor, Mr Buswell today came out swinging on the "flawed plans".
He said Labor's estimated $730 million cost for the railway to Perth Airport fell well short of what was needed with Treasury advice obtained through the Public Transport Authority estimating it would cost $1.595 billion.
He said Labor's proposed route around the southern runway posed significant engineering challenges, including building under the water table.
"They have effectively promised a railway line with funding that will not deliver even half of the project," he said.
"It simply cannot be done. They have got it wrong. This is in my view a big embarrassment for them."
However in what has emerged as an election costings stand off Mr Buswell would not release the Treasury analysis, despite saying he planned to expose more "big holes" in the Metronet plan over the course of the election campaign.
He said Labor should submit the Metronet plan to Treasury for analysis next Friday after Treasury releases the pre-election financial statement on Thursday.
Under caretaker conventions Treasury has three days to provide a response to Labor which should be made public, Mr Buswell said.
However Labor leader Mark McGowan responded by demanding Mr Buswell release the costings on Metronet that he obtained from Treasury "at taxpayer expense".
Labor has costed its five priority projects at $3.8 billion.
Mr Buswell has said they would cost $6.4 billion and stood by that figure today.
"They will have to explain how they're going to fund it," Mr Buswell said.
Mr McGowan, who today confirmed $107 million in funding on making country roads safer, said Mr Buswell's refusal to release the costings showed he was not telling the truth.
"We will see what Mr Buswell does," he said.
"But he claims now that he has costed our policy twice. He can release that information if he so chooses considering it is paid for by the taxpayers and he should and I call on him to do that.
"We'll consider his proposal."