The Treasurer Troy Buswell has attacked the Opposition's transport costings, accusing Labor of embarrassing itself by underestimating its figures.
Mr Buswell has asked the Public Transport Authority to put a price on the Opposition's pledge to build a rail line to the Perth Airport.
The project is one of the commitments making up Labor's $3.8 billion Metronet plan.
Labor has costed the airport link at $731 million but Mr Buswell says the true figure would be closer to $1.6 billion.
"They have got it wrong.
This is a big embarrassment for them," he said.
"I don't think the Opposition's costings have given any consideration to the extreme engineering and technical difficulties around taking a railway line around the southern end of the airport.
"That means there's a $860 million black hole.
This is what happens when the Labor party adds up or develops these figures on the back of an envelope in Ken Travers' office." The Labor leader Mark McGowan has hit back at Mr Buswell's claims.
"We are very confident of our costings," he said.
"I don't think anyone takes anything Mr Buswell says seriously.
"He claims now that he's costed our policies twice.
"He can release that information, considering it was paid for by the taxpayer and I call on him to do that.
"I am quite concerned that through his actions Mr Buswell has compromised those departments and I am concerned he has abused the public service." In what's shaping up as a political stand off, Mr Buswell said he's not yet prepared to release the PTA's advice but challenged Mr McGowan to submit his plans for assessment.
"If they are so comfortable with that, next Friday, the day after Treasury hand down the pre-election financial statement, they can send that plan to Treasury for it to be costed," he said.
"That is the process that applies under the caretaker convention." But Mr McGowan says it's up to Mr Buswell to make the first move.
"Mr Buswell's the one with all the information at his fingertips.
Let him release that information and then we'll see exactly what's going on here."