WA Premier rubbishes Grylls treasury claim

The West Australian premier has rubbished claims by Nationals leader Brendon Grylls that his controversial mining tax policy was first proposed by treasury.

Mr Grylls told the West Australian that his policy proposal to increase mining lease rent paid by BHP and Rio Tinto from 25 cents to $5 per tonne was initially raised by the WA Department of Treasury.

But Premier Colin Barnett told reporters on Monday that was not the case, and any discussion by treasury about the tax was held after Mr Grylls had publicly announced the policy.

Mr Barnett said he did not know what was motivating Mr Grylls and did not agree with his proposal.

"For all of my political career I've been around the resources industry and one of the most sacred aspects of that is that you do not tear up contracts and state agreements are basically contracts."