Detained Australian writer Dr Yang Hengjun has insisted he is innocent of the espionage allegations made against him and told Chinese authorities he has never been a spy for any government. But his isolated incarceration in Beijing – potentially facing the death penalty – remains indeterminate.

Yang, a former diplomat turned globally influential blogger and advocate for democracy, was arrested when he arrived in China on 18 January this year. He is being detained by the Ministry of State Security, charged with one count of espionage. There is no indication when, if at all, he will face court.

An Australian citizen, he has not been allowed access to his lawyers or to see family members.

Yang has not been given any letters written to him by family, nor allowed to write to them. He has been shown photographs of his family, but was not allowed to keep them.

However, his interrogations have been reduced to once a month, and he no longer has his hands and feet shackled in irons during questioning.

Sources have confirmed to the Guardian that Yang’s only interrogation this month began at midnight, and lasted for two-and-a-half hours. Yang was not restrained for this interrogation but reported the lines of questioning were often unrelated to the allegations against him, and, in some cases, “absurd”.

Sources have also confirmed Yang’s health has deteriorated over the past month. He suffers from high blood pressure and several other serious health conditions which are being monitored and medicated but have worsened significantly in detention. He is allowed to shower once a week, and has access to a small enclosure outside his cell – with access to fresh air and natural light – for one hour, twice a day.

In detention, Yang has continued to maintain his innocence but has said he suspects someone in the investigation team is trying to frame him for espionage and is seeking to incriminate him.

He has told interrogators he has never been a spy for any government or any organisation.

Chinese-born Yang has said he was an “ordinary public servant” for the Chinese government, serving in the ministry of foreign affairs for a decade until 1997, when he moved to the US to work as an independent researcher. Since then, he has not been involved with any government anywhere in the world.

Yang said he believed in the rule of law in China, although he was losing confidence in this. He said he has felt threatened at times, but did not elaborate on the nature of these threats.

The Guardian understands none of Yang, his legal team, or the Australian embassy have been told the specific charges against him, or presented with any evidence of his alleged offending.

The Criminal Law of the PRC details punishment for espionage and endangering national security.

Collusion with foreign states to harm China’s territorial integrity, sovereignty or security attracts a prison term of at least 10 years.

“State organ personnel”, such as diplomats, who defect and endanger national security can be jailed for between five and 10 years.

But crimes that endanger national security, that have caused “particularly serious harm to the country and the people”, and where “the circumstances are particularly vile”, can attract the death penalty.

Yang was formerly a diplomat for China’s ministry of foreign affairs, before working in the private sector in Hong Kong and moving to Australia, then to the USA. A writer of spy novels, he has been a popular blogger, political commentator and agitator for democratic reforms in China for more than a decade.

He was known online as a “democracy-peddler”, and regarded around the world as an authoritative and influential political commentator on China.

Yang, who became an Australian citizen in 2002, had been living in the United States, where he was a visiting scholar at Columbia University, before flying to Guangzhou with his family in January. His wife and child were able to enter China, but authorities escorted Yang from the plane into detention.

He was initially held under a system known as “residential surveillance at a designated location”, a type of secret detention of up to six months in which authorities can deny a suspect access to lawyers and to family, and restrict external communication. In July, Yang was moved to a Beijing detention centre in the lead-up to being charged.

He is allowed one half-hour consular visit a month.

The Guardian understands Yang’s case remains in the investigation period, which could last until March of next year.

The Australian government has repeatedly pressed China that Yang be treated in accordance with international human rights law, “with special attention to those provisions that prohibit torture and inhumane treatment, guard against arbitrary detention and that protect the right to freedom of thought, conscience and religion”.

In a speech this week, the Australian foreign affairs minister, Marise Payne, said: “I will continue to advocate strongly for fair and transparent treatment” for Yang.

Yang’s former doctoral supervisor in Australia, Dr Feng Chongyi, said there appeared to be no evidence Yang has ever been engaged in espionage, and that the charges against him were politically motivated.

“I’m encouraged by the speech by the foreign minister. I still hope that the Chinese authorities make the commonsense decision and release Yang because there is no evidence to support any sensible charge against him.”

Feng, an associate professor in China Studies at the University of Technology, Sydney, told the Guardian: “He’s no longer shackled, and the number of interrogations has reduced, I would say this is due to international attention, including pressure from the Australian government”.

In August, a spokesman for China’s foreign ministry, Geng Shuang, said Yang’s arrest had been handled in accordance with Chinese law and that the Australian citizen was in good health.

Geng also warned Australia against “interference” in the case.

“China deplores the Australian statement on this case … the Australian side should earnestly respect China’s judicial sovereignty and must not ... interfere with a Chinese case,” he said.