John Zakhariev, 21, has been in custody since September in a case that his family fears is being used by the eastern European country to grandstand on counter-terrorism.

It threatens to become a major diplomatic mire similar to that of Sydney man Jock Palfreeman, who is serving 20 years for murder in a case marred by corruption and tainted evidence.

Palfreeman and Mr Zakhariev are in the foreigners' wing of Sofia's maximum security prison.

Bulgarian authorities allege that Mr Zakhariev tried to join a terrorist group in Syria in 2013 but was knocked back due to a lack of combat training.

They allege he returned home to skill himself before heading to Bulgaria in 2016 because training would be cheaper there.

They allege he attended shooting ranges and had "jihadist material" with him, alleged to be a book on the history of Islam.

They have suggested he "ran away" from Australian authorities and obtained a Bulgarian passport to safeguard himself.

Mr Zakhariev, the son of a Vietnamese-Australian mother and a Bulgarian-Australian father, grew up in Sydney's inner west and graduated from Waverley College in 2012.

His 82-year-old father Svetlomir, a former UN diplomat, had been helping his son through the ordeal but died two weeks ago.

In an interview aired on Bulgarian television last year, Mr Zakhariev said he converted to Islam in high school but reverted to Christianity in mid-2016.

"When I became a Muslim it was a very emotional time in my life," he said. "I just broke up with a girlfriend and I had problems with my family ... And it seemed to me at the time that Islam offered something which I really wanted which was companionship and friendship. I have to admit I never really fully believed in Islam."

His lawyer, Hristo Botev, told Fairfax Media his client joined the Street Dawah proselytising group in Sydney in 2013, led by Mohammad Ali Baryalei, who went on to become a high-ranking Islamic State recruiter.

With the help of Mohamed Zuhbi, another Dawah member who was later killed in Syria, he flew from Sydney to Turkey via South Korea and was escorted over the border.

"I was very distressed seeing the images coming out of Syria and the stories," Mr Zakhariev said in his TV interview. "I wanted to go to see what the situation was, to see what I could do to help the Syrian people."

He stayed in a house controlled by Jaish al-Muhajireen wal-Ansar, not a proscribed terror group at the time, but said he left after eight days after witnessing the jihadists' brutality.

He said he continued on a planned holiday across Europe before returning home to a sociology degree at the University of Sydney.

He tried to travel to Bulgaria in 2015 but was stopped at Sydney Airport and questioned by ASIO. His passport was taken off him but returned two weeks later.

He then travelled to Bulgaria in 2016 to obtain a Bulgarian passport and to help his father, who was moving back to his homeland to retire, to settle in.

He said he visited a legal shooting range in Sofia but there was nothing sinister in it and he wanted a Bulgarian passport so he could travel and work in the EU.

His older sister, Nevena Zakhariev, a 25-year-old arts teacher, said her brother had already made plans to enrol in tourism studies in Canada or the US.

However, four months later, he was arrested.

"At first, I just thought it was a big misunderstanding, we laughed it off thinking he'd be home in a couple of weeks because he's done nothing wrong," she told Fairfax Media.

"But as months and months passed, it became more serious. My fear is that the government wants to make an example out of him and he'll go to jail for something he didn't do."

Mr Botev said the allegations that he was training to join Islamic State were "absurd" and came at a time of huge counter-terrorism spending in Bulgaria.

"I firmly believe this is a poorly attempted entrapment operation by the police in order to justify the millions they have received from [the EU in] Brussels to fight against terrorism," he said.

Compounding the family's distress, Ms Zakhariev said the Australian government had been unhelpful, only sending infrequent reports to say he is in good health when, in reality, her brother told her he had been assaulted in prison.

"Everything I ask them, they tell me to go through his solicitor. Even small things like, how can I visit my brother?"

She questioned why her brother, whom she described as a deep thinker interested in world affairs, was only arrested three years after the trip to Syria and despite witnesses coming forward to corroborate his version of events.

His mother, Anne Ngo, visited him in Sofia Central Prison in February despite struggling with stage four cancer.

"He'd said he was doing OK because he had Dad and Jock there but now Dad's died, he's worried," Ms Zakhariev said.

The Australian Federal Police said it would not comment on individuals and a DFAT spokesman would only confirm it was providing "consular assistance to an Australian detained in Bulgaria, in accordance with the Consular Services Charter".

The charter stipulates that the Australian government can't intervene in legal matters, provide legal assistance or lobby for favourable treatment. It's understood Mr Zakhariev's last consular visit was March 6.

He has a court hearing on March 17 and faces eight years in jail if convicted.