A close friend of Yang's, Associate Professor Dr. Feng Chongyi told DW he believes Yang is likely being held in "special detention," known amongst those in the Chinese legal profession as "black jail."
Professor Feng remains in close contact with the author's family. "They are not allowed to say anything about the case," he told DW.
The professor works in the China Studies department at the University of Technology, Sydney, where he was Yang's PhD supervisor between 2005 and 2009. Since this time, he says they have remained "very close," organizing conferences and editing books together.
Feng was himself detained in China in 2017 for a week but says he was "much more lucky than [his] student."
"The Chinese authorities handled me in a more professional way. The interrogations were probably similar [to Yang's] but they allowed me access to my mobile phone and to the outside world."
"When the Chinese government takes someone, it's a very well-planned act," Feng said. "It was a miracle I was let go after one week. Yang Hengjun faces much more serious prospects."
Professor Feng says the Chinese government holds people in "special detention" when it's preparing to lay down the charge of espionage. Also known as "residential surveillance at a designated location," this form of detention was added to China's Criminal Procedure Law in 2012 and it allows authorities to hold someone in a secret location and deny them access to lawyers.
"If he's not lucky, they can even deny access to consular support," Feng explained. "It's horrible because this detention can take up to six months or even longer. He's likely subject to torture. People are forced to confess to crimes they did not commit, which they will use to press charges later on."
The professor says these instances of detention, although more common for Chinese dissidents has in recent years begun to apply to foreigners — especially overseas Chinese.
"For Yang Hengjun, although he is an Australian citizen, the Chinese government still maintains some imperialist mentality so they view him as a subject of the Chinese government because he's of Chinese heritage."
Yang Hengjun started his career as an official in the Hainan Province Foreign Affairs Office in the 1980s.
He wrote the spy novel trilogy Fatal Weakness which touched on sensitive political issues and therefore was published in Hong Kong and Taiwan but not China.
He then became a prolific blogger and in 2005, at the height of his blogging career, had 15 blogs and over 50 WeChat groups, according to Feng Chongyi.
Yang moved to Australia at the turn of the century and in 2005, wrote his PhD under the mentorship of Professor Feng on the topic of citizen journalism, specifically the struggle between Chinese government control and "netizens." Feng blames himself for Yang's arrest, having nurtured his writings as a dissident on the internet.
"It was the beginning of his struggle, the beginning of his trouble when he became a political blogger."
Although he has scaled down his political blogging activity over the last few years, his reputation as a citizen journalist — or, as Feng puts it, a "democracy peddler" — remains. "He's very well connected with all different groups including within the government, dissidents and other writers," Feng said.
"Every blogger is a modern-day Tiananmen Square," Yang said at the forum in Bonn. "The Chinese authorities have continued to increase internet censorship over the last few years — even with the support of western companies."
Professor Feng has called on the international community to put pressure on China to release Yang Hengjun, saying it would be dangerous to set a precedent for what he calls a "kidnapping."
"The international community and democracies around the world need to act together to prevent the Chinese government from establishing this kind of precedent — that they can take someone as a hostage for political purpose," Feng told DW.
"Now they take Australians. They already took Canadians. They can take anyone … Germans, French for the same purpose."
The Australian government has called for "further clarification" on Yang's detention, urging Beijing to treat the writer fairly. Foreign Minister Marise Payne said Australian diplomats met Chinese officials on Thursday and would continue to "seek an explanation of the possible charges against him."