Sue and Len Roberts-Smith appeared in the Federal Court on Monday to support their son, who was accused of being a war criminal in reports co-authored by journalist Nick McKenzie for Nine-owned newspapers.
A judge in 2023 found the reports were proven on the balance of probabilities.
But in his appeal of that decision, Roberts-Smith argues there was a miscarriage of justice because McKenzie allegedly unlawfully obtained details about the former soldier's legal strategy.
Leaked audio of a conversation between McKenzie and a witness was tendered to the Federal Court on Monday in which he says Roberts-Smith's ex-wife and her friend had been "actively briefing us on his legal strategy".
"I shouldn't tell you," McKenzie says in the taped call.
"I've just breached my f***ing ethics in doing that, like this has put me in a s*** position now."
In a statement outside court, Mr and Mrs Roberts-Smith said the conversation was "extremely concerning on a number of levels" and questioned whether McKenzie's conduct led to an unfair trial.
They reiterated their support for their son who they say has been subject to vilification by the journalist and Nine for almost a decade.
Roberts-Smith's barrister Arthur Moses SC told the Federal Court the journalist engaged in wilful misconduct that affected the outcome of the trial.
Mr Moses pushed for a retrial before the full bench of the Federal Court to eliminate the possibility of apprehended bias that may arise if the matter was heard by a single judge.
He noted former silk Nicholas Owens SC - who represented Nine newspapers during the initial trial - was recently appointed as a Federal Court judge.
A decision about amending the appeal is expected to be handed down on Monday afternoon.
Roberts-Smith rose to prominence in 2011 after he was awarded Australia's highest military honour, the Victoria Cross, for single-handedly taking out machine-gun posts to protect pinned-down colleagues in Afghanistan.
His reputation, however, was tarnished in 2018 after McKenzie's explosive reports alleging the special forces veteran was complicit in the murder of four unarmed men during his deployment in Afghanistan.
In June 2023, Justice Anthony Besanko found the reports had been proven on the balance of probabilities - a lower standard than in a criminal proceeding.
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