The Federal Court has ordered that Mr Palmer and Mr McGowan attend mediation in person and "participate in a bona fide attempt to achieve settlement of the matters in dispute".

Justice Richard White has further ordered that the mediation take place before the end of September with the mediator to be selected by agreement between the parties.

If no agreement can be reached, a mediator will be appointed by the court.

The orders follow indications last month that Mr Palmer was willing to participate in mediation in his defamation action.

At that time, Mr McGowan indicated he presently "does not wish to participate" in the negotiation process.

Mr Palmer last year filed a lawsuit against the premier, claiming his public comments, including labelling him the "enemy of West Australia", had damaged his reputation.

Mr McGowan lodged his own defamation counter-claim a month later.

Justice White previously noted the contrasting positions of the two parties on mediation and said it was "improbable" the matter would go to trial until the first quarter of 2022.

A separate High Court matter between Mr Palmer and the state of Western Australia is scheduled to be heard on June 15.

It relates to extraordinary legislation passed in WA's parliament in August to amend a 2002 state agreement with Mr Palmer's Mineralogy company and terminate arbitration between the two parties.

The bill is designed to block Mr Palmer from claiming damages from the state.

Mr Palmer is seeking almost $30 billion from WA taxpayers over a decision by the former state government not to assess one of his mining projects.

The federal government recently filed a notice of intervention rejecting Mr Palmer's position.

The NSW and the Northern Territory have also intervened in support of WA.

The defamation case will return to the Federal Court on October 1 for a further case management hearing.

Australian Associated Press