Western Australian Transport Minister Dean Nalder has quit Cabinet out of frustration at Premier Colin Barnett's "erratic and illogical" decision making, intensifying the State Government's leadership crisis.

Key points: Dean Nalder joins Tony Simpson in stepping down from the frontbench

Dean Nalder joins Tony Simpson in stepping down from the frontbench Mr Nalder will consider standing for the leadership if there is a spill

Mr Nalder will consider standing for the leadership if there is a spill He says he had grown increasingly frustrated at Mr Barnett's "erratic and illogical" decisions

Mr Nalder joined Tony Simpson in stepping down from the frontbench, in a move he says was not coordinated but which leaves the Government in disarray six months out from the state election.

He told the ABC he would not initiate a spill motion at Tuesday's Liberal partyroom meeting but would consider standing for the leadership if someone else did.

"If people in the partyroom felt that I was the right person, then it's something I would definitely consider," Mr Nalder said.

"But it's not a primary driver for me."

He claimed the final straw had been when Mr Barnett instructed him not to proceed with his preferred option of a tunnel for stage two of the $1.9 billion Perth Freight Link heavy haulage road project to Fremantle Port.

"I've been trying over the last three to four weeks to sit down with the Premier to try and work out a way forward and I haven't been able to and I just don't believe I can perform the function of the minister in these circumstances," Mr Nalder said.

He said a tunnel would have saved several hundred million dollars compared to an above-ground road but Mr Barnett had told him he was doing too many things and "we didn't have a mandate to do it".

Mr Barnett's office has been contacted for comment.

Mr Nalder was also worried about the state's economy, with the budget beset by record debt and deficit levels.

"I'm very keen to see a narrative developed that actually demonstrates to people that we understand what's happening out in the economy and we can do things to help," he said.

He told News Corp he had grown increasingly frustrated at Mr Barnett's "erratic and illogical" decisions.

Mr Barnett has been dogged by leadership speculation for much of this year and was recently forced to insist that despite continuing tension within the Liberal Party, no-one was pressuring him to hand over the reins to his deputy, Liza Harvey, before the March 2017 election.

However, some senior Liberals were understood to be deeply unhappy with the Premier's decision to retain Mr Nalder in Cabinet after the Transport Minister was linked to a privately-funded poll showing the Government losing office at the state election.

'Government's direction no longer serving people of WA'

Mr Nalder's decision to quit came hours after Mr Simpson advised Mr Barnett of his resignation from the ministry, effective immediately.

"I have taken this step reluctantly and with a heavy heart, but it has come to the situation that the Premier's leadership and the Government's direction is no longer serving, I think, the people of Western Australia," he said in a media conference.

Mr Nalder said he was surprised to hear Mr Simpson had resigned but refused to say when he learned his former frontbench colleague was quitting.

Mr Barnett issued a statement saying he was sorry to see Mr Simpson leave, but wished him well.

Liberal backbencher Ian Britza said the Premier had also lost his support.

Mr Britza said he could vote for Mr Nalder or Corrective Services Minister Joe Francis in a spill motion, but had lost faith in the Premier.

"I've had the thoughts for quite a while, but when my electorate every now and again begin to voice their disapproval of my party and my leader, all of a sudden you start to think 'I've actually thought that but all of a sudden my electorate is saying it,'" he said.