Australian Federal Police officers have launched a formal investigation into the fraud allegations which have forced Peter Slipper to step down from his role as Speaker of the House of Representatives.

Mr Slipper's former staffer James Ashby has accused him of misusing Cabcharge vouchers.

Police say they have been assessing the allegations since they became public more than a week ago, and have decided they warrant further investigation.

The AFP says investigators have already spoken with a number of potential witnesses in relation to the allegations, but says it will not be making any more comment at this stage.

Mr Slipper has denied the claims.

The police move comes as Opposition frontbencher Christopher Pyne comes under increasing pressure to reveal the extent of his contacts with Mr Ashby, who is also taking civil action against Mr Slipper for alleged sexual harassment.

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Fairfax newspapers have reported that Mr Pyne, the manager of Opposition business, shared drinks with Mr Ashby in the Speaker's parliamentary office a month before Mr Ashby went public with his allegations.

The papers say Mr Pyne drank with Mr Ashby from about 9:30pm until after 11:00pm on March 19, and quotes Mr Pyne as saying he was merely "passing the time of day".

Reports say that after the meeting Mr Pyne called the Speaker's office requesting Mr Ashby's mobile phone number.

Mr Pyne has confirmed he met Mr Ashby three times in the months before Mr Ashby made his allegations, but says the Government is trying to turn that into a fanciful "political conspiracy".

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"The more contacts I have in the Speaker's office, the better," he told ABC Local Radio in Adelaide this morning.

"But I've never spoken to James Ashby on the phone and I've never texted him on the phone."

"On no occasion did [Mr Ashby] raise the matters canvassed in the Federal Court action that have subsequently come to light with me," he said in an earlier statement.

On ABC1's 7.30 last week, Opposition Leader Tony Abbott said he was not aware of Mr Ashby receiving any assistance from the Coalition in preparing his court claim.

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But the Government has seized on the meeting to suggest a double standard in the Opposition's attack on Mr Slipper.

Treasurer Wayne Swan challenged Mr Pyne to release phone records proving he had not been contacting Mr Ashby to assist with the civil sexual harassment case against the Speaker.

Today, Trader Minister Craig Emerson says Mr Pyne has been "caught out trying to deceive the Australian public".

"Last night Mr Pyne was saying he can't recall seeking the contact details of Mr Ashby after having met with Mr Ashby and been drinking with Mr Ashby for almost two hours," he said.

"It has emerged that indeed Mr Pyne did seek the contact details of Mr Ashby. He did so through a text message and email.

"Mr Pyne must stand up in front of the Australian media and explain why he has changed his story between yesterday and today."

Dr Emerson says Mr Pyne needs to explain who was aware and what role they played in the preparation of the harassment claims against Mr Slipper.

"I think these are legitimate questions to ask on behalf of the Australian people," he said.

The scandal has forced Mr Slipper to stand aside from the Speaker's chair, potentially weakening Labor's already fragile hold on the House of Representatives.