Federal MP Peter Slipper has questioned why police have suspended their investigation into whether former Howard government minister Mal Brough was involved in a conspiracy to bring him down.

Labor MP Graham Perrett wrote to the Australian Federal Police (AFP) in December, asking them to investigate allegations Mr Brough was part of a campaign to harm Mr Slipper and the Federal Government.

He made the request after a Federal Court judge threw out a sexual harassment case against Mr Slipper, declaring it to be an abuse of process and designed to further the political interests of Mr Brough and the Liberal National Party (LNP).

It emerged during the court proceedings that Mr Brough had requested extracts of Mr Slipper's diary from the staff member who ultimately filed the sexual harassment case, James Ashby.

Mr Ashby has sought leave to appeal against the ruling.

In a letter sent to Mr Perrett, AFP Commander Errol Raiser says the investigation into Mr Brough's actions has been suspended because of the ongoing legal action.

"The AFP is aware that an appeal has been lodged with the Federal Court to be heard on May 30, 2013," the letter states.

Mr Brough has been preselected by the LNP for Mr Slipper's seat of Fisher. ( AAP: Steve Gray )

"As the facts surrounding the appeal are consistent with your referral, the AFP has suspended its evaluation of the matter until the outcome of the appeal has been finalised."

Mr Perrett says he is disappointed by the decision, given that the September 14 election is now less than 200 days away.

"It's really drifting on, and obviously in terms of investigating any trails, they will get colder and colder and staler and staler," he told ABC News Online.

"I'm comforted by the fact that they obviously see this as serious, but I'm troubled by the fact that they've suspended their investigation.

"A three-month delay is quite significant.

"We've got an election on September 14 where Mr Slipper's credibility and good name with his voters is significant."

Mr Perrett says he will be writing back to the AFP, arguing that parts of the investigation could continue despite the appeal because Mr Brough is not party to the ongoing legal action.

Concern

In a statement to the ABC, Mr Slipper says it is clear that the AFP are taking Mr Perrett's complaint very seriously, which must be a concern for those members of the LNP who support Mr Brough.

But he has questioned why the police investigation has been suspended, given Mr Brough is not involved in the application to appeal.

"To my knowledge, (Mr) Brough and (former staffer Karen) Doane... are not seeking to contest the judge's adverse findings against them and thus have not sought leave of the court to challenge His Honour's findings," he said.

"In these circumstances it is difficult to understand why the AFP has suspended/postponed (though not cancelled) its investigation of the matters raised in Mr Perrett's letter to the AFP Commissioner.

"It will be interesting to see how long the LNP is prepared to tolerate unresolved matters concerning Mr Brough.

"I would be very surprised if Mr Brough is still the LNP candidate at the election to be held on September 14."