The State Opposition has demanded the Government release costings for its light rail plan, suggesting the figure could be more than $2 billion.

The State Government wants to build a 22 kilometre line, dubbed the Metro Area Express, to link Mirrabooka and the CBD.

The Transport Minister Troy Buswell has previously said the project would cost more than a billion dollars but he will not give exact details until the election campaign officially begins.

Labor's transport spokesman Ken Travers says that is not good enough.

"It's now clear that the cost could be up around $2 billion," he said.

"If we want to have sound financial planning coming into the state election, it is absolutely important that the Barnett government make that clear.

"It's about trying to hide the truth from the people of Western Australia."

Mr Buswell has accused the Opposition of hypocrisy, saying Labor refuses to detail costings for its own big ticket promise promise, the 'Metronet' heavy rail project.

"When we examine the components of the information we have available that make up the Labor Party plan, the cost is $6.4 billion," he said.

"If they go out into the election and offer any costing that is less, they will not be truthful in attempting to build what they've committed to."

"I think the public would expect if you want to draw up a map that conveniently links marginal seats around Perth, you'll explain how you're going to pay for it."

The Opposition denies Metronet would cost that.