A former chairman of the Australian Red Cross who had been awarded the Order of Australia for services to charity, Mr Wansley, 58, was shot dead by a gunman on the back of a motorcycle while employed by Deloitte Touche Tohmatsu to investigate financial malpractice at Thai sugar mills.

The case that lingered through Thai courts for 16 years raised serious questions about the country's judicial system.

Thailand's Supreme Court on Wednesday reversed the death penalty for sugar mill executives Somchok Suthiwiriwan and Sompong Buasakul, handing them life sentences.

The court upheld an earlier sentence of life imprisonment for Boonpan Suthiwiriwan, another mill executive.

The men were employees of a sugar mill in Nakhon Sawan province owned by the family of wealthy provincial businessman Pradit Siriviriyakul.

Mr Pradit was initially charged with conspiracy to murder but was acquitted by a three-judge panel which rejected police testimony he offered a $4 million bribe for the investigation to be dropped.

The panel accepted evidence that Mr Pradit told an employee "it was good the foreigner is dead" but found he made the comments after the murder.

The Australian government made repeated representations to Thailand during the case and Deloitte appointed former National Party leader and deputy prime minister Tim Fischer to act as an adviser amid doubts the Thai courts would deliver justice.