The WA Treasurer Troy Buswell has denied directing Treasury to hide a controversial budget decision involving the WA Insurance Commission but says he cannot be sure if any of his staff did.

When legislation allowing the Government to take up to 65 per cent of the commission's profits was introduced in June, Mr Barnett told Parliament the decision had only been made after the March election.

But the Opposition has accused the Premier of lying to Parliament, saying it has since received supplementary information from the Government indicating the decision was actually made last year.

It also accused the government of trying to cover up the move in December's mid-year review, saying while expected revenue from the bill was booked in the document, it was not specifically itemised.

Mr Buswell has denied any cover-up but says while he did not direct Treasury to hide the decision, he cannot be sure no one else did.

"There's a lot of people in Government," he said.

"What I know is, I didn't have any conversation with [the under-Treasurer] Mr Marney or others.

"People from within my office, within the Premier's office, more than likely other ministerial offices, have a whole range of conversations with Treasury in the lead up to particularly the mid-year review and the Budget.

"I'm not privy to those conversations."