Western Australia's Treasurer claims to have exposed a further billion dollar black hole in Labor's Metronet rail costings.

The Opposition has estimated the plan at $3.8 billion but Troy Buswell says it would actually cost $6.4 billion.

Mr Buswell says part of the difference is due to Labor not factoring in the cost of upgrades to the Midland line he says would be needed to cope with three proposed new services to the airport, Ellenbrook and a northern line.

"The cost to deliver that duplication is estimated by the Public Transport Authority at $1.1 billion," he said.

"$1.1 billion dollars of a rail upgrade that the Labor Party's plan will require to the existing rail networks."

Those extra costings amount to $4.9 billion.

Mr Buswell says the remaining $1.5 billion gap includes $800 million for technical works for upgrades at the Perth airport link.

He says he will release further details of the costings during the campaign.

"This is another example of an unaffordable, unachievable plan that will never happen," he said.

The Labor leader Mark McGowan has rejected Mr Buswell's claim and is standing by his figures.

"I wouldn't trust Mr Buswell as far as I could I throw him," he said.

He is repeating calls for Mr Buswell to release the PTA's information.

Labor's transport spokesman Ken Travers says Mr Buswell's claims of duplication costs are baseless.

"This is another attempt by Troy Buswell to muddy the waters," he said.

"It is simply false and the PTA has made it very clear in previous documents that they've released, that you can operate 20 trains an hour on our existing rail network.

"With improved signalling, 30 trains an hour."