A prominent Australian former SAS soldier launched an appeal Monday after losing a defamation case that implicated him in a string of war crimes in Afghanistan.

Ben Roberts-Smith, who was a member of Australia's elite Special Air Service regiment, sued three Australian newspapers over stories printed in 2018 that alleged he was involved in the killings of unarmed Afghan prisoners.

The ex-soldier lost the multi-million-dollar courtroom battle in June 2023, with a judge finding the newspapers had proven that the bulk of their allegations were "substantially" true.

Roberts-Smith, 45, attended the first day of the appeal hearings in Sydney, wearing a dark suit, white shirt and light grey tie as he entered the Federal Court building.

His lawyers argued in the appeal that the judge "erred" in the way he assessed some of the evidence.

The trial is scheduled to run over two weeks.

Before the defamation trial, Perth-born Roberts-Smith had been Australia's most famous and distinguished living soldier.

He won the Victoria Cross -- Australia's highest military honour -- for "conspicuous gallantry" in Afghanistan while on the hunt for a senior Taliban commander.

The Age, The Sydney Morning Herald and The Canberra Times shredded this reputation when they published a series of bombshell reports alleging that Roberts-Smith had engaged in a pattern of criminal and immoral behaviour while serving overseas.

The papers reported Roberts-Smith had kicked an unarmed Afghan civilian off a cliff and ordered subordinates to shoot him.

He was also said to have taken part in the machine-gunning of a man with a prosthetic leg, later bringing the leg back to an army bar and using it as a drinking vessel with comrades.

The court ruling ultimately implicated Roberts-Smith in the murder of four unarmed Afghan prisoners.

He has not been charged in a criminal court over the allegations.