WA Opposition Leader Mike Nahan is stepping down following months of speculation and mounting pressure, saying it was always his plan to hand over the Liberal leadership before the next state election.

Key points: Mike Nahan is quitting as Opposition Leader but says he will stay in Parliament for now

Mike Nahan is quitting as Opposition Leader but says he will stay in Parliament for now He says he will "look back at a job well done" after facing down his critics

He says he will "look back at a job well done" after facing down his critics Liberal sources say deputy Liza Harvey is the frontrunner take over the leadership

"I have decided to step down because I am confident we are in a good position with a number of leadership aspirants and after the federal election we are in a good position," Dr Nahan told ABC News.

"Right from the start when I took up the position, I always decided that halfway through I would review my position with the aim to handing over to new leadership.

"I was not warming the chair, I was an active leader. After a loss, our largest in history, someone had to step in and lead the team."

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Dr Nahan, who served as treasurer under the previous Barnett government for three years, took over as party leader after the Liberals' thumping 2017 election defeat.

He was the only Liberal MP willing to take on the role, but was forced to doggedly defend his position from the beginning, at one stage accusing colleagues of trying to "white-ant" him.

The 68-year-old American-born MP said the West Australian newspaper had been a particularly harsh critic of his leadership.

"People know that after a loss like that [in 2017], the Opposition Leader is not often the final one," he said.

"The West Australian has been particularly bent on my leadership.

"Everyone says this is the hardest job in politics. It is hard. You have a lot of critics, you have a lot of cynics, and you have to really mould a team and motivate them and convince them to believe that you can win.

Mike Nahan is standing down from the job he took on after the 2017 WA election. ( ABC News: Andrew O'Connor )

"I think I have done that. I look back at a job well done.

"In this job you have to put the noise aside and focus on the task."

Nahan had 'guts' to take on job: Premier

Despite attempts by the party to soften his quirky image, which earned him the nickname "Ned Flanders", murmurings about Dr Nahan's viability as leader never ceased.

Dr Nahan says it is his intention to see out this term in Parliament. ( ABC News: Andrew O'Connor )

He came under renewed scrutiny last year when he revealed he was still a US citizen because of an unresolved tax dispute with the country's Internal Revenue Service (IRS).

WA Premier Mark McGowan took full advantage of the ongoing leadership speculation and Dr Nahan's perceived weakness, labelling him "the worst Opposition Leader I have ever seen".

"I have seen Paul Omodei, I have seen Troy Buswell — you leave them for dead," Mr McGowan teased across the chamber during Question Time last year.

But even the Premier paid his respect to Dr Nahan earlier this week, amid growing speculation the Liberal leader's resignation was imminent.

"I will say this about Dr Nahan. He had the guts to take on the Opposition Leadership when the others wouldn't," Mr McGowan said.

Premier Mark McGowan says Dr Nahan was prepared to step up when others wouldn't. ( ABC News: Benjamin Gubana )

"He was prepared to step up when Mr [Dean] Nalder, Mr [Sean] L'Estrange or Ms [Liza] Harvey wouldn't. And they have sat there over the last two years undermining him.

"At least Dr Nahan had the courage to take on the role."

Liza Harvey frontrunner to replace Nahan

Dr Nahan would not be drawn on who would replace him as leader, saying it would be a decision for the party room later this week.

A party room meeting is expected to be held on Thursday morning to elect a replacement leader.

Liberal party sources have told the ABC deputy leader Liza Harvey was the frontrunner take over the leadership.

Liza Harvey is among those in the frame to replace Dr Nahan as Liberal leader. ( ABC News: Benjamin Gubana )

Earlier on Wednesday, Mrs Harvey told reporters the party was not talking about leadership and labelled members that had leaked information to the media "anonymous cowards".

Dr Nahan said it was his intention to see out this term in Parliament, but would not say whether he would re-contest the next election.