The 33-year-old Australian, whose family is based in Newcastle, has been granted parole after serving almost 12 years of a 20-year sentence in Bulgaria for murder.

His father Simon Palfreeman, a Newcastle pathologist, met the decision with happiness but also caution.

"I'm just going to keep my powder dry [on discussing his release] until I know a bit more about what's going on."

While he is understandably wary of pre-empting his son's release, it has been reported that the appeal court's decision is final.

He did say that his son was "an incredibly strong-willed character".

"He's been in there for 12 years. Lesser people probably would have succumbed to all sorts of things," he said.

His son had worked to defend the human rights of other prisoners, starting the Bulgarian Prisoners' Rehabilitation Association.

"He's a passionate character who believes very strongly in social justice."

Jock spent six months living and working in a regional town in Bulgaria in 2006.

"He really liked the people and got on very well with them," Simon said.

"He basically was helping them on farms and various things. He just really enjoyed the lifestyle."

In 2007, he joined the British Army. The barracks closed down over Christmas, so he returned to Bulgaria to visit friends.

On a night out, he went to the aid of a Gypsy boy being attacked by 15 to 20 soccer hooligans. The group included Bulgarian law student Andrei Monov, who was fatally stabbed. Another man was wounded. It was not illegal in Bulgaria to carry knives.

Jock admitted he had a knife, but alleged he was being attacked. The Bulgarians claimed he committed murder.

In the book Every Parent's Nightmare, journalist Belinda Hawkins wrote that Monov was the only son of two people well connected in the legal fraternity.

The case had "many twists and turns, vital evidence was kept out of court and crucial witnesses never called".

Julian Burnside QC argued that inconsistencies in witness statements created a reasonable doubt that would have led to acquittal in an Australian trial.

Jock's case has gained support over the years from high-profile figures including actors Rachel Ward and Richard Gere, along with Geoffrey Robertson QC.

Simon said it was horrendous to think that his son had spent his "formative years of 21 to 33 in any sort of prison, let alone a prison that's overseas".

"It's been pretty traumatic for the whole family."