The Australian Federal Police says it doesn't have enough evidence to prosecute former Liberal minister Mal Brough for copying former speaker Peter Slipper's diary.

In an application to the Federal Court on Thursday Andrew Berger, barrister for the AFP, said investigators wanted access to all text messages and phone calls between Mr Brough and Mr Slipper's former media advisor James Ashby.

"What [the AFP] can get so far isn't enough to bring the investigation to a head," Mr Berger said.

The AFP needs the court's permission to use evidence from a civil case in a criminal prosecution. In this case, the evidence includes hundreds of sometimes lewd text messages sent and received from Mr Ashby's phone in 2012 that were used in his case against Mr Slipper for sexual harassment.