Mr Turnbull and Mr Joyce appear to have split over WA Nationals leader Brendan Grylls' proposed iron ore tax hike.

While Mr Joyce was a leading critic of federal Labor's now-defunct mining tax and has close ties to mining magnate Gina Rinehart, he appears to have an open mind on Mr Grylls' controversial plan.

"Minister Joyce said the proposal was a matter for the state government and he would not run down any proposal that was designed to get a better deal for its constituents," said a spokesman for Mr Joyce, who is currently acting PM.

WA Labor opposes the plan, in a reversal of the political positions taken on Kevin Rudd's mining tax in 2010.

The apparent split has been exposed by the West Australian newspaper. Miners BHP and Rio Tinto are said to be infuriated by the proposal that would increase from 25¢-a-tonne to $5-a-tonne the "production rent" on iron ore, bringing in an estimated $7.2 billion over four years.

Speaking in Laos this week Mr Turnbull told the newspaper that Mr Grylls' plan could damage investment in WA.

"It obviously sends a very troubling message to mining companies and other people considering making long-term investment," he said.

"It's a state issue, of course, but we view with great concern, as does the whole business community, the imposition of substantially increased taxes on particular, nominated companies."

Federal Resources Minister Matt Canavan - a Nationals MP - has said he has "grave concerns" about the proposal.

Mr Joyce called Labor's mining profits tax a "debacle" and a "shambles" and even criticised respected senior public servant Ken Henry for proposing it in his comprehensive tax reform review.