Mr Ashby, who is suing Mr Slipper and the Commonwealth, claims the stood-aside Speaker made unwelcome advances and sent him sexually suggestive text messages after Mr Ashby joined his staff in December last year. Mr Slipper and the Commonwealth allege Mr Ashby's case was intended to ''vilify'' Mr Slipper and ''destroy or seriously damage'' his reputation and career. They are seeking an order that proceedings be stopped as an abuse of process.

Yesterday, Mr Ashby's barrister, Michael Lee, SC, used the spectre of criminal charges to successfully argue that his client should not have to put forward his defence to claims his sexual harassment suit against Mr Slipper is an abuse of process.

Further, he may have committed the offence of causing harm to a public official under the Commonwealth Criminal Code. That offence carries a maximum 10 years' jail.

The letter, written on June 28, also suggests Mr Ashby could be prosecuted for participating in a conspiracy with Mr Brough, Lewis, and another Slipper aide, Karen Doane.

Mr Lee said his client had been accused of serious criminal offences and had a right to claim a privilege against self-incrimination. ''We want to proceed in the way common law trials have for centuries,'' he said. ''We don't propose to put on a jot or tittle of evidence until we've seen what gets in against us.''

Lawyers for the Commonwealth and Mr Slipper opposed the application, arguing Mr Ashby should be forced to at least address the areas which did not raise a risk of self-incrimination.

David Chin, for Mr Slipper, said the Speaker was not asking the court to find Mr Ashby criminally liable as part of its application for the sexual harassment case to be thrown out.

However, Justice Steven Rares agreed Mr Ashby could not be forced to incriminate himself and excused him from putting on any evidence or revealing his defence until the case against him had been presented in full.

Justice Rares also said the amount of time and money being spent on the case ''seems to have got out of all proportion'' compared to the original claim for damages made by Mr Ashby.