A Beijing court is holding a closed-door trial for Australian journalist and former TV anchor Cheng Lei, in a high-profile diplomatic case.

Ms Cheng is accused of illegally supplying state secrets overseas. Her family maintains she is innocent.

The Chinese-born Australian was working for Chinese state media outlet CGTN prior to her detention in August 2020.

Canberra has repeatedly raised concerns over her detention and has called for "basic standards of justice" to be met.

Little is known about the exact nature of Ms Cheng's alleged offences and it's not clear when a verdict will be handed down.

Speaking to reporters outside of the courtroom on Thursday, Australia's ambassador to China Graham Fletcher said he and other Australian officials had been denied entry into the hearing. Chinese courts often bar outsiders from trials deemed politically sensitive.

"This is deeply concerning, unsatisfactory and regrettable. We can have no confidence in the validity of the process which is conducted in secret," he said, adding that Australia would continue to "advocate strongly for Ms Cheng Lei's rights and interests".

Ms Cheng, who was previously reported to have suffered ill health while in detention, was now "doing okay" considering the circumstances, said Mr Fletcher. Ms Cheng's lawyer had also told Australian media that she was in good health and good spirits.

Mr Fletcher added that Australia has requested that Ms Cheng be allowed to speak to her children.

"They haven't had any contact with their mother since the detention," he said.

A family statement released to Reuters on Wednesday said her children and elderly parents "miss her immensely and sincerely hope to reunite with her as soon as possible".

Prior to her detention, Ms Cheng had worked in Beijing for several years. Many of her family members, including her two young children, live in Australia.

In August 2020, she suddenly disappeared from television and could not be contacted by friends or relatives. Her employer, CGTN - the English-language channel of the state broadcaster - also wiped its websites of Ms Cheng's profile page and work.

China initially announced she was being held on national security grounds, and in February last year authorities formally arrested Ms Cheng on spying charges. Her family have said they have no idea why she had been detained.

Australia said its representatives had been able to visit her once a month - in line with the bilateral consular agreement with China.

Relations between Australia and China have deteriorated in recent years, leading to diplomatic and trade ructions.

Analysts say this has made it harder for Australian authorities to negotiate with Beijing over their citizens' release.

Beijing has arrested or expelled a number of journalists in recent times. Shortly after Ms Cheng's arrest, the last two Australian media correspondents in China fled the country following consular advice.

Another China-born Australian citizen, writer Yang Hengjun, has also been in detention in China for three years. He denies espionage charges, and is said to now be in ill health.