Australian Federal Police (AFP) have ramped up their investigation into the copying of former speaker Peter Slipper's diary.

The ABC has been told the AFP has made an application to the Federal Court, which, if granted, would allow police to use evidence from an earlier civil case in a criminal prosecution.

That evidence includes text and picture messages exchanged between Mr Slipper's former staffer, James Ashby, and Liberal National Party MP Mal Brough.

Mr Brough is under investigation by the AFP, who executed a search warrant at his Queensland home last year as part of that probe.

Mr Ashby launched a sexual harassment suit against Mr Slipper in 2012.

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During that case it emerged Mr Ashby had made copies of Mr Slipper's diary, which were then sent to Mr Brough and former News Limited journalist Steve Lewis, with the intention of using them to show Mr Slipper had misused his Government-issued Cabcharge vouchers.

Mr Brough unseated Mr Slipper as the member for the federal Queensland seat of Fisher at the 2013 election.

Now the AFP has asked the Federal Court to grant it permission to use the material harvested from Mr Ashby's phone, including excerpts from Mr Slipper's diary sent by Mr Ashby to Mr Brough.

In its application, the AFP has said the messages will be used in the current investigation, and any subsequent prosecution.

Permission is necessary because the exhibits were originally only to be used in the civil case.

Mr Ashby's sexual harassment suit was rejected by a Federal Court judge, who said Mr Ashby had been part of a "combination" including Mr Brough, which had used the legal action as a political weapon against Mr Slipper.

Mr Ashby then appealed and the original judgement was set aside, but Mr Ashby later dropped the case.

However, it subsequently emerged the AFP is investigating whether Mr Brough committed a crime by encouraging Mr Ashby to copy pages of Mr Slipper's diary.

Mr Brough stood down from his role of Special Minister of State in December, saying he would not contest the upcoming federal election.