A West Australian Liberal partyroom meeting has begun in Perth to debate a leadership spill motion aimed at ousting Premier Colin Barnett from the state's top job.

Supporters of Mr Barnett have insisted he will comfortably see off the leadership challenge, with a number of high-profile ministers backing the Premier as they walked into the meeting.

But backers of former transport minister Dean Nalder argued it would be much closer, believing nearly 20 MPs are behind a motion to oust Mr Barnett and others remained undecided.

Twenty-four of the 46 Liberal MPs need to back the spill motion, which will be moved by former corrective services minister Murray Cowper at the 10:00am (AWST) meeting, to topple Mr Barnett from the position he has held for eight years.

Entering the meeting, Mr Barnett said it was a "new experience" to face a leadership challenge, but he was confident he had the support of MPs.

Mr Cowper's foreshadowed spill motion follows months of leadership speculation, which reached a climax at the weekend when Mr Nalder and Local Government Minister Tony Simpson declared they no longer supported Mr Barnett and resigned from Cabinet.

Mr Barnett was confident as he entered the meeting. ( ABC News: Andrew O'Connor )

Mr Cowper told 720 ABC Perth he felt instability and a lack of direction in the party needed to be addressed.

"It needs to come to a head ... so that we can stop the bleeding, so to speak, and get on with the business of running the state," he said.

He said Mr Barnett had promised to step aside if he was "tapped on the shoulder".

"It's clear to me that he's not going to do that and subsequently there are a number of people who feel disaffected," Mr Cowper said.

"The only way to deal with this issue at this particular point in time is to have a count and we'll see what happens.

"This is not about Mr Nalder, this is about clearing the air, getting people to stand up and see which way they fall."

Concern over Premier's leadership style

Mr Nalder spent much of Monday arguing his case for change, saying Mr Barnett had lost his way as leader and the community had stopped listening to him.

"There are concerns in the party room and I believe they should be discussed in the party room," he said.

"What will occur at the back end of that, I'm not sure.

Mr Nalder's backers say he is close to having the numbers. ( ABC News: Lauren Day )

"I have concerns about his leadership and leadership style, particularly the administration that sits around him in the Premier's office."

Deputy Premier Liza Harvey, who is viewed by many in the party as the most viable leadership alternative, yesterday ruled out running even if the spill motion was successful.

Mr Nalder's backers privately believe the spill motion is within striking distance of having sufficient numbers, a claim rubbished by those behind Mr Barnett.

On their way in to the partyroom meeting today, Mental Health Minister Andrea Michell and Speaker Michael Sutherland were among those backing the Premier.

Attorney-General Michael Mischin said he expected the spill motion would easily be defeated, and said he would not be backing it.

MP Liz Behjat was scathing about those behind the spill motion.

"Rats and cockroaches have to go," she said.

"Strong leadership is what we have with Colin Barnett and that's what we are staying with."

Meanwhile, Prime Minister Malcolm Turnbull has thrown his support behind Mr Barnett.

Speaking in New York, Mr Turnbull said Mr Barnett had a fantastic record spanning the past eight years.

"While the decisions of his partyroom are decisions for its members, they all understand what a great leader he has been ... what he offers and will continue to offer the West Australian people, and unity is everything in politics."