Labor frontbencher Craig Emerson has accused Tony Abbott of using "weasel" speak to outline what his party knew of the sexual harassment allegations against Speaker Peter Slipper.
Mr Slipper stood aside as speaker of the lower house of parliament on Sunday pending a possible criminal investigation into allegations he had misused taxpayer-funded dockets.
The besieged MP also faces a civil lawsuit that claims he sexually harassed a male staff member.
Dr Emerson said on Saturday the allegations of criminality against Mr Slipper for misusing taxpayer-funded dockets had become "absurd".
Mr Slipper, who is alleged to have signed several Cabcharge vouchers that were later filled out by a limousine driver, attempted to clear his name this week by releasing copies of 13 Cabcharge vouchers he said were signed and completed in his handwriting.
Dr Emerson believes the dockets are authentic.
"On the face of it, given that the allegation is that they were blank but signed, these are not blank, they are signed and they call into serious question the allegation of criminality against Mr Slipper," he told ABC Radio.
He said it was unfair to criticise Mr Slipper for vague trip descriptions on the dockets like 'suburbs to suburbs'.
"If there's a new standard which says that if Cabcharge has 'suburbs to city', 'city to airport', then that is a fraud against the Commonwealth - then just about every MP is guilty of a fraud against the Commonwealth," he said.
"That's the level of absurdity at which this has now gone."
Mr Slipper also faces the civil lawsuit over claims he sexually harassed a male staff member.
When Opposition Leader Tony Abbott was questioned about the allegation, he said he had no specific knowledge of any involvement by the Liberal party in assisting the man who lodged the claim, former staffer James Ashby.
Dr Emerson was sceptical of Mr Abbott's use of the word "specific", which he referred to as "weasel" speak.
"What did Mr Abbott really know?" he asked.
Workplace Relations Minister Bill Shorten refused to be comment on whether Mr Slipper would resume his role as Speaker when parliament resumed next month.
The claims against him were yet to be proven, he said.
"The Prime Minister has indicated there is a process underway ... we support our Prime Minister,'' he said.
However, his opposition counterpart Eric Abetz said everything surrounding Mr Slipper resembled "one big rat's nest''.
It was unlikely he would return to the Speaker's chair, he said.
"It is very difficult to foresee a circumstance where he could return ... there has been so much swirling around the Speaker, that it would be very difficult for him to fully recover from that which has already been disclosed,'' he told the ABC.