An Australian academic jailed in China on espionage charges fears he will die in prison if he does not receive medical treatment, his friends said Monday.

Chinese-born Australian Yang Jun has been jailed in China since 2019, accused of spying in a closed trial that was heavily criticised by human rights activists.

Yang's ailing health has been caused by a large cyst growing on his kidney, which has come to the attention of Australian diplomats who have visited him in detention.

"If something happens with my health and I die in here, people outside won't know the truth," he said in a note shared recently with friends and supporters.

"If something happens to me, who can speak for me?"

Political scientist Feng Chongyi, who was detained and interrogated by Chinese authorities in 2017, said he had grown increasingly concerned about his friend's health.

"These prisons, or detention centres, are unable to provide decent medical treatment," he told AFP.

"It deeply worries me. When they talk about having a major operation to remove the cyst, that may endanger his life," he added.

Yang, who denies the spying claims, was arrested on a rare trip back to China in January 2019.

The writer and academic -- who also goes by his pen name Yang Hengjun -- has previously told supporters he was tortured at a secret detention site and fears forced confessions may be used against him.

His closed-door trial was heard in Beijing in mid-2021, with Yang still awaiting the verdict.

A spokesperson for Australian Foreign Minister Penny Wong said on Monday that the government had "deep concerns" about the "ongoing delays in his case".

Canberra's improved relationship with Beijing has ignited a renewed push to free Yang as well as jailed Australian journalist Cheng Lei -- who also faces vague espionage charges.

Cheng recently described her bleak prison conditions in a note dictated to Australian officials from her cell.

"I miss the sun," read the message, described as a "love letter" to Australia.

"In my cell, the sunlight shines through the window but I can stand in it for only 10 hours a year."

Cheng has been detained since August 2020, but was only formally arrested in February 2021.

She was tried last March behind closed doors, with Australia's ambassador to China blocked from entering the court to observe proceedings.

Attempts to raise the plight of Cheng and Yang have irked Chinese officials, who have previously urged Australia to refrain from meddling in the judicial system.