The Australian Federal Police will be allowed to use text messages and phone calls between former speaker Peter Slipper and his former media adviser James Ashby in its criminal investigation into former Liberal minister Mal Brough after a court found "there is a legitimate public interest".

On Thursday, Federal Court Justice Geoffrey Flick granted an application for leave by the federal police to use evidence adduced in Mr Ashby's sexual harassment case against Mr Slipper in deciding whether to prosecute Mr Brough for copying the former speaker's diary.

Peter Slipper, Mal Brough and James Ashby. Credit:Alex Ellinghausen, Harrison Saragossi, Wolter Peeters

The federal police had said it didn't otherwise have enough evidence "to bring the investigation to a head".

Investigators will be allowed to use two CD-ROM discs dated April 24, 2012, containing hundreds of sometimes lewd text messages sent and received from Mr Ashby's phone in 2012. The discs were attached to an affidavit filed on behalf of Mr Ashby by Rodney McKemmish, an expert in computer forensics, in his suit against Mr Slipper.