The Opposition's attack on beleaguered Labor backbencher Craig Thomson has been blunted after Opposition Leader Tony Abbott refused to say if he would sack a Liberal MP facing similar allegations.

Mr Thomson has admitted he authorised credit card payments for prostitutes while secretary of the Health Services Union in 2005.

But he denied it was him who visited the escort agency, said his signature had been forged on credit card receipts and said another man, who he would not name, had paid back $15,000 to the union.

It has also been reported the Labor Party paid $90,000 towards a defamation action Mr Thomson took against the Fairfax newspapers which originally published the allegations.

The Opposition says if the payment had not been made, Mr Thomson would have faced bankruptcy and lost his central NSW seat of Dobell, thus bringing down the minority Federal Government.

It is also reported that Mr Thomson used the union credit card to secure more than $100,000 in cash - allegedly to bankroll his election campaign for Dobell.

He also reportedly reduced a woman to tears at a weekend rally on the central coast, but has said the abuse claims are "complete rubbish".

Mr Thomson has been under sustained attack from the Opposition, which polls indicate would win an election forced by his resignation.

And Prime Minister Julia Gillard has twice been forced to defend her backbencher in parliament, saying he has her full confidence.

But Mr Abbott - saying he does not want to "set himself up as a moral paragon" - declined to agree that Mr Thomson should have to resign.

Asked twice on Radio 2UE if he would sack a Liberal backbencher under similar circumstances, Mr Abbott refused to agree.

"Hopefully, I would not find myself in that situation but the truth is that people should not misuse money," he said.

"Members of parliament should not be abusive with members of the public. Political parties should not give money to members of parliament for dodgy defamation actions, and this has just got trouble written all over it."

He said people should expect that they are represented by MPs of "basic integrity".

"I don't know all the details of this case because I'm sure the Labor Party and the Prime Minister is hiding a lot from us, but on the face of it it looks very bad," he said.

But he indicated no-one, not even himself, was above reproach.

"The one thing I don't want to do is set myself up as a moral paragon because you never know what tests you might face. You just never know these things," he said.

Asked why the issue was even being tolerated, Mr Abbott said it was a question for the Prime Minister.

"She's got someone who looks to be in diabolical trouble, but she can't even say that," he said.

"She's got to say that she's got full confidence in the guy because obviously if there's any problem with this fellow she's got a difficulty, perhaps a terminal difficulty, in the Parliament.

And despite repeated questioning, Mr Abbott would not say if Mr Thomson should be sacked.

"What should the Prime Minister be doing here? The Prime Minister should be conducting herself with a degree of political integrity which, I'm afraid, has been lacking from polling day," he said.

Liberal Senator Michael Ronaldson has called for Mr Thomson to be sacked.

"The only thing standing between Craig Thomson and bankruptcy is the Australian Labor Party, with loans and payments being made, and we know the only thing standing between Craig Thomson and section 44 of the Constitution is the payments by the Australian Labor Party," he said.

"If he were a bankrupt he would no longer be entitled to be a member of this place, this Australian parliament. That is the only thing standing between Mr Thomson and his rightful ejection as an elected representative of this country."