There is growing pressure on West Australian Treasurer Troy Buswell to step down over his misuse of parliamentary entitlements to carry on an affair with Fremantle Greens MP Adele Carles.

Mr Buswell is in a tenuous position following revelations that he misused his government car and credit card during his affair with Ms Carles.

It is not the first time Mr Buswell has been in trouble over his government entitlements. Twice last year he was forced to repay the Government because of incorrect travel expense claims.

Premier Colin Barnett defended Mr Buswell at the time, but there is growing speculation that Mr Barnett will not be so forgiving now.

State Opposition Leader Eric Ripper says the community has lost faith in Mr Buswell and he must be sacked.

"If Mr Buswell has misused taxpayers' funds, the Premier will have no alternative but to sack him," he said.

"Misusing taxpayers' funds, particularly for a treasurer, particularly when there's form, is a sackable offence.

"I have been reluctant to comment on what I have regarded in the past as a private matter, [but] when taxpayers' funds are misused the matter becomes public. That's the issue now, the misuse of taxpayers' funds."

The WA nurses union says it is appalled Mr Buswell has not been sacked in light of the dismissal of more than seven nurses and a number of doctors for falsely claiming meal and entertainment expenses last August.

WA secretary of the Australian Nursing Federation, Mark Olson, says the only difference between Mr Buswell's situation and the nurses is that he still has his job and they do not.

"If Mr Buswell stays then those nurses should be reinstated. Any other action by the Premier would cast the simple label of hypocrisy on his government," he said.

A spokesman for Mr Barnett says reports that Mr Buswell has been sacked are wrong.

Mr Barnett is spending the long weekend at his country property, east of Perth, and will make an announcement about Mr Buswell's future tomorrow.

Affair revelations

Earlier today Mr Buswell revealed that he had had a four-month affair with Ms Carles, but denied that it had any impact on his job.

"I can confirm that Adele Carles and I had an affair for almost four months which began in late December. This affair recently ended, at which time I advised my family and the Premier of my actions," Mr Buswell said.

"My family and I have been working through this matter privately since then. I did not want this matter to become public because of the impact this publicity would have on my wife and my children."

Ms Carles made the affair public at the weekend with a lengthy statement expressing regret at the pain the relationship had caused both families.

The statement followed weeks of rumour and speculation of an affair between the pair who are both married with children.

In an emotional press conference Mr Buswell apologised to the Premier and his wife, confirming that he misused public funds on four occasions during the secret liaison.

"I can confirm that my ministerial car dropped me off to meet Adele Carles on three occasions. I will repay the Government garage for the cost of those trips," he said.

"In mid-February I travelled to Albany in the afternoon, ahead of a series of meetings the following day. I was joined by Adele Carles that evening. I will repay the cost of the accommodation."

Mr Buswell confirmed he is seeking professional help but this is not the first time he has made national headlines.

Two years ago he admitted to sniffing the chair of a female staff member and snapping the bra of a Labor staffer.

Ramifications

Emeritus Professor David Black of Edith Cowan University says despite this latest scandal, it is unlikely Mr Buswell will be sacked by Mr Barnett.

"Clearly he has built up a litany of errors which normally would have led to his resignation or to him being asked by the Premier to leave the ministry, but the problem all along and the problem still now is that he's such a key part of the Government, in a sense indispensable," he said.

But Mr Buswell's long-term future may not be so rosy. He was once touted as a potential leader of the WA Liberal Party, and now Professor Black says that ambition has been severely damaged.

"For the moment he's staying, but clearly his whole political future has been very seriously clouded and one can only assume that... somewhere down the track all the chickens will come home to roost."

Mr Buswell says his relationship with Ms Carles did not impact on his decision making as Treasurer.

"This relationship has not had an impact on the discharge of my duties, nor has it influenced me in my dealings with issues related to the Fremantle electorate or in regard to issues advanced by the Greens in Australia," he said.

But the affair has raised questions about Ms Carles's voting patterns in the Lower House, says Professor Black.

"One would have to say that that's obviously if she wanted to survive as a Member for Fremantle in the next state election - which is normally a strong Labor seat - she would need to be able to appeal across the political field, if you like," Professor Black said.

"She couldn't just get votes from Green supporters and from dissident Labor people, so in that sense she's entitled to perhaps a say, to exercise a relatively independent vote.

"But the fact is her vote was a little surprising on a couple of issues in relation to the stance that the Greens normally take."

Elected in 2009, Ms Carles is the Member for Fremantle and the first Greens MP to be elected to the WA Lower House.

Rumours of her affair with Mr Buswell had been around for some time and it is understood she went public at the urging of her Greens colleagues.

But Ms Carles may pay a high price for her public admission. Professor Black says her future as a Greens MP hangs in the balance.

"I think her future in politics - both as a Green and the member and certainly for a very difficult seat to win - must be extremely unlikely now."

WA Greens Leader Giz Watson did not return PM's calls this afternoon.