The One Nation senator Malcolm Roberts will be cross-examined about whether he knew he was a British citizen and the case challenging his eligibility may also scrutinise his sister’s evidence, the court of disputed returns has heard.

At a hearing in Canberra on Friday the chief justice, Susan Kiefel, appointed a friend of the court to test Roberts’s evidence and made orders for Roberts to submit expert legal advice on British citizenship law.

The high court, sitting as the court of disputed returns, will hear legal argument on 10 October about the eligibility of Roberts, the deputy prime minister, Barnaby Joyce, senators Fiona Nash, Nick Xenophon and Matt Canavan, and former Greens senators Larissa Waters and Scott Ludlam.

Roberts’s counsel, Richard Scheelings, told the court the One Nation senator intended to bring expert evidence to show his effort to renounce his British citizenship by email on 6 May 2016 was effective despite not being accompanied by a fee.

Scheelings said Roberts had preliminary advice from British barrister Adrian Berry, a junior to the commonwealth’s expert, Laurie Fransman, but was seeking a more senior barrister.

Kiefel said the process of finding an expert “should have been in train some time ago” and said the court’s “timetable had not been adhered to in a number of respects” by Roberts.

Kiefel suggested that the commonwealth and Roberts’s experts should confer on exactly how their opinions differed to confine the legal issues in question, noting they “shouldn’t have far to walk” as they worked at the same chambers.

Justice Patrick Keane will hear cross-examination of Roberts and the two experts at a hearing on Thursday. Kiefel appointed senior counsel Stephen Lloyd as a friend of the court to test Roberts’s evidence at that hearing.

The solicitor general, Stephen Donaghue, said he anticipated Lloyd would need to test Roberts’s evidence about his knowledge that he was a British citizen.

He also suggested Lloyd may need to cross-examine Roberts’s sister, who has given evidence supporting Roberts’s case, and to scrutinise what steps Roberts took to renounce his citizenship.

In the case of Barnaby Joyce, Tony Windsor will be the “contradictor”, arguing Joyce was not eligible for election.

Donaghue submitted that the cases of senator Matt Canavan, Fiona Nash and Nick Xenophon were “materially indistinguishable” from Joyce’s, because they were all born in Australia and say they had no knowledge they acquired foreign citizenship by descent.

Donaghue said it was “almost inevitable” those four cases would “stand or fall together”, although Kiefel noted the evidence was distinct in each case.

Windsor intends to bring evidence about French, German and United States citizenship law to show that the issue of citizenship by descent was understood to be a disqualification under section 44 of the constitution, possibly even at the time of federation.

Larissa Waters, who resigned along with her colleague Scott Ludlam, will submit that she was ineligible, but the commonwealth will argue that she was not disqualified by section 44. No other contradictors were appointed by the court.