New South Wales police say they are looking at whether there are grounds for an investigation into allegations levelled against Federal Labor MP Craig Thomson.

Shadow attorney-general George Brandis wrote to NSW Police Commissioner Andrew Scipione yesterday asking for police to look at Mr Thomson's use of a union credit card.

Senator Brandis wrote that Mr Thomson had given implausible explanations on how his Health Services Union credit card had paid for escort services.

Mr Thomson has admitted authorising the payment to an escort service in 2005, but has said another man used the card and forged his signature.

He has consistently denied any wrongdoing.

Senator Brandis wrote that "The overwhelming weight of evidence... suggests that Thomson himself received, signed for and authorised (sic) payment for the services".

"I submit there is a strong prima facie case that he has committed one or more offences against the Crimes Act," Senator Brandis wrote.

"I draw your attention, in particular, to the offence of larceny; fraudulent appropriation; larceny by a clerk or servant; and fraud."

Senator Brandis said he considered the clearest offence to be fraud, "where a person who by any deception dishonestly obtains property belonging to another or obtains any financial advantage".

He said the maximum penalty for fraud was 10 years imprisonment.

In a brief statement, police say the correspondence has been referred for internal assessment to determine whether a criminal offence has occurred.

The statement says normal assessment and investigation protocols will be followed.

Senator Brandis welcomed the decision, but said that as the matter was now in the hands of the police, he would not be commenting further.

He said that it was "not primarily a political matter. It is primarily a legal matter".

New allegations

Earlier, Prime Minister Julia Gillard was forced to defend Mr Thomson amid new allegations which appear to cast doubt on his denial that he used his union credit card to pay for prostitutes.

At the time the credit card was used for prostitutes, phone calls to the escort agency were made from Mr Thomson's mobile phone but he has denied all allegations that he visited prostitutes.

Now court documents published by The Sydney Morning Herald have revealed the mobile phone used to call the escort agency was also used to call political and union contacts within the ALP on the same day - April 7, 2005.

The NSW Supreme Court documents also show Mr Thomson's card was used to pay $2,475 to an escort agency on April 8.

The documents appear to contradict Mr Thomson's contention that someone else had used his credit card and, by implication, the mobile phone.

The NSW Labor Party has paid a reported $90,000 to help pay Mr Thomson's legal expenses for a defamation action he took against Fairfax newspapers - money the Opposition says prevented Mr Thomson being declared bankrupt and losing his central coast seat of Dobell.

A by-election would be likely to cost the Government its slim majority in Parliament.

'Sickening'

Yesterday in Parliament the Opposition's finance spokesman, Andrew Robb, ramped up the attack on Mr Thomson, saying the Government's support for Mr Thomson is "sickening".

"Look at the evidence that comes out daily. Look at what we saw today from Fairfax, that he has lied; that he is a thief. Yet the Prime Minister stands up here daily and supports it," Mr Robb said.

Sorry, this audio has expired Prime Minister speaks to AM ( Sabra Lane )

This morning on AM, Ms Gillard deflected questions about why Labor had paid the legal fees, saying it is a "question for the NSW branch of the Labor Party".

"Decisions by the NSW branch of the Labor Party are their decisions."

And she tried to turn the pressure back on the Opposition, describing its position as "hypocrisy" and referring to a shoplifting charge being faced by Liberal Senator, Mary Jo Fisher.

"There is only one member of Parliament who's been charged with a criminal offence - it's not a member of my team."

Ms Gillard conceded she has talked to Mr Thomson about the allegations, but again deflected questions saying she wants to wait until an investigation by Fair Work Australia is complete.

"In the meantime, the Member for Dobell is doing his job as a local member in this Parliament."