A Chinese-Australian writer who has been detained in China for more than four years said he fears he could die in jail, after a large cyst was found on his kidney.

Yang Hengjun, an Australian citizen and democracy activist, was detained in 2019 during a visit to see family in China and charged with espionage -- accusations Yang has denied.

A Beijing court held his trial in secret in 2021 but a verdict has been repeatedly delayed.

In a message to supporters last Thursday, Yang, 58, said he had experienced discomfort and pain in his kidney for a couple of months.

After a recent medical examination, a doctor advised Yang he has a 10-centimeter cyst squeezing his kidney, according to the message.

"If something happens with my health and I die in here, people outside won't know the truth," Yang said in the message. "If something happens to me, who can speak for me?"

Feng Chongyi, Yang's friend and former PhD doctoral supervisor at the University of Technology, Sydney, called for the Australian government to work with the international community to pressure the Chinese government to release Yang on medical parole.

"Yang Hengjun has been subjected to arbitrary detention in China by Chinese secret police for more than four years and his life is in danger," he said.

"His real 'crime' is his advocacy for universal values such as democracy, human rights and the rule of law."

Asked about Yang's case at a news conference Monday, Australian Prime Minister Anthony Albanese said he would raise concerns about Yang's health when meeting with Chinese leader Xi Jinping at the Group of 20 summit in New Delhi next month.

"We always raise issues of Australian citizens when either myself or my Ministers meet with our international counterparts, including China," Albanese said.

Yang, who was born in China, previously worked as an official with the Chinese Foreign Ministry.

Though he holds Australian citizenship, Yang spent most of his time in the United States.

Before his detention, he routinely posted satirical commentaries critical of the Chinese government to his nearly 130,000 followers on X, previously known as Twitter.

Yang is one of two Australians currently detained in China. Cheng Lei, an Australian TV anchor who worked for China's state broadcaster CGTN, is accused of illegally supplying state secret overseas and has been detained for three years.