Friends of an Australian writer detained by China on suspicion of spying fear his deteriorating health could see him eventually die behind bars.

Yang Hengjun has again pleaded his innocence ahead of the third anniversary of his arrest by Chinese authorities at Guangzhou airport in January 2019.

“Sometimes, I’m pessimistic and sometimes, I’m optimistic — I’m confident I didn’t do what they said I did,” Dr Yang said in a message conveyed to supporters before Christmas from a Beijing prison.

Supporters say they are extremely concerned about the democracy advocate's worsening condition and warn the jailed Chinese-born Australian citizen is being denied proper medical treatment.

The 56-year-old is believed to be suffering severe problems with gout, high uric acid, high blood pressure, impaired vision, and dizzy spells.

Recent blood tests have revealed rising levels of creatinine, prompting fears Dr Yang could suffer kidney failure, but a Chinese prison doctor has been unable to provide any treatment or advice on treatment.

Writer calls for Beijing to reveal details of his case 

Last year Dr Yang faced a one-day secret trial in Beijing, but relatives and Australian diplomats were denied access to the closed-court hearing because of apparent COVID-19 restrictions and national security grounds.

A verdict in the case has again been delayed until at least April, with supporters urging he be immediately released on bail for medical treatment, fearing he could die in prison like fellow Chinese dissidents including Liu Xiaobo.

As he awaits a decision by a Chinese judge, the Australian citizen is demanding local authorities "open my case and publish it" to "provide details to the world, the Australian Government, and the country".

"We should apply to open the case, and you can see for yourself. They said it's about espionage. I hope it's just about Chinese judicial corruption."

"I have seen news that China is smashing corruption in the legal system. This is just one instance of it," Dr Yang has told family and supporters.

Australian diplomats have had limited access to Dr Yang since he was detained, and he is currently being held in a crowded cell with no sunlight, where lights are kept on all night and detainees share a communal, open toilet and sleep on a hard floor with not enough room to stretch.

The Australian government has repeatedly protested against China's secret handling of Dr Yang’s espionage charges, accusing Beijing of "arbitrary detention" of the writer.