The State Opposition has promised to set up a parliamentary budget office if it wins the March election.

The Labor leader Mark McGowan says the office would be able to prepare independent costings on party promises and policies, both inside and outside of the election caretaker period.

Mr McGowan says it would take the politics out of costings and ensure the public is given accurate, detailed information.

He has promised a Labor government would set up the independent office within the Department of Treasury during its first term.

"We've seen a significant debate over the course of this election as you do every election in relation to these issues," he said.

"And, I think the public is sick of it, I think what the public would like to see is some certainty around the costings process and both sides have access to an independent parliamentary budget office."

The Premier Colin Barnett has rejected the idea, saying it is unnecessary.

"Treasury is best able to cost them because bear in mind, if you're making cost estimates you need to get engineering advice, financial advice, whatever it might be that goes into that," he said.

"And, I don't understand why Mark McGowan is questioning the integrity and the professionalism of Treasury, I have full confidence in Treasury."