Australian writer Yang Hengjun is expected to be formally charged with endangering national security, according to his lawyer.

Yang, a Chinese public intellectual who has long advocated for democratic reforms in China, has been detained for the last six months in an unknown location in China.

Yang’s lawyer, Mo Shao Ping, said Yang’s family in Shanghai has been told to come to Beijing to receive a formal notice of the writer’s criminal detention, indicating formal charges are forthcoming.

The maximum punishment for endangering national security is life in prison or in serious cases, the death penalty. The notice the family receives should have more details of the charges Yang faces, which could include additional charges. Yang was initially detained on suspicion of endangering national security.

“Anything is possible. Right now we cannot say,” Mo said.

The Australian embassy in Beijing said a relative of Yang’s notified the embassy that he has been transferred to a detention centre in Beijing.

Yang, a naturalised Australian citizen since 2002, has been held under a system known as “residential surveillance at a designated location”, RSDL, a type of secret detention where authorities can deny access to lawyers and family.

At the end of six months under RSDL, a suspect should be released, formally charged, or transferred to criminal detention. Friday marks six months since Yang’s detention under RSDL, according to his lawyer.

Yang’s case could complicate already cooling ties between Australia and China, over concerns about potential Chinese interference in national affairs, Huawei and human rights.

On Monday, Australia’s foreign affairs minister, Marise Payne, said her government was “deeply concerned” about Beijing’s treatment of Uighurs, an ethnic minority in Xinjiang. Payne said China had blocked Australia’s attempts to offer consular assistance to dual citizens and their families believed detained in Xinjiang.

Intelligence officials believe Chinese hackers may have been responsible for a security breach at Australian National University in June.

Yang, a former Chinese diplomat, is a popular writer and blogger known for his spy novels and political commentary.

Being transferred to criminal detention does not augur well for Yang, human rights advocates say. Friends of his previously believed he would be released following the 30th anniversary of the June Fourth crackdown.

“While we don’t know the details of Yang’s case, the Chinese government has a record of deploying vague ‘national security’ charges to prosecute peaceful critics,” said Yaqiu Wang, China researcher for Human Rights Watch.

This week, Geng Shuang, a spokesman for China’s ministry of foreign affairs, said Yang’s case was proceeding “in accordance with the law” and that the state had “fully guaranteed” the protection of his rights.

Critics say the pace of Yang’s case shows a lack of evidence on the part of authorities.

“They haven’t got evidence,” said Feng Chongyi, an academic at the University of Technology Sydney and a friend of Yang’s. “If they had gotten anything they would have gone immediately to arrest him and would have charged him earlier before the expiry of six months.”

Feng believes how the Australian government responds will affect how Chinese authorities pursue Yang’s case.

“They continue to do the so-called investigation. If there is no cost to them of course they will take this kind of political prisoner as long as they can.”

Yang’s wife, Yuan Xiaolong, a permanent resident of Australia, has been barred from leaving China.

Human Rights Watch urged the Australian government to press Chinese authorities to release her from the exit ban.

Yang is known in online circles as minzhu xiaofan, or “democracy peddler”. “I’m like an old auntie jabbering on, always promoting democracy and repeating its benefits,” he wrote in an article in 2014.

“Dictatorship is always torn down in one night, but good democracy isn’t built in one night.”

Additional reporting by Lillian Yang