The Chinese Government has moved to formally charge Australian citizen Yang Hengjun over an ill-defined espionage allegation more than a year after first detaining him, ABC can reveal.

Key points: Dr Yang has been detained for more than 420 days over as yet unexplained allegations relating to espionage

Dr Yang has been detained for more than 420 days over as yet unexplained allegations relating to espionage Australian Foreign Minister Marise Payne has previously said Dr Yang's ongoing detention is "unacceptable"

Australian Foreign Minister Marise Payne has previously said Dr Yang's ongoing detention is "unacceptable" Dr Yang is now almost certain to now face trial in China

The news brings to an end Australian attempts to have the writer and democracy activist returned to Australia before he is fully enmeshed in the byzantine workings of Beijing's judicial system.

It is also a signal that Beijing has not been swayed by the efforts of the Australian Government to have Dr Yang released.

Australian Foreign Minister Marise Payne had previously said Dr Yang's ongoing detention was "unacceptable", and he should be released or at the least afforded basic standards of justice and humane treatment.

Chinese Ministry of Foreign Affairs officials said Australia should not interfere in the case.

Dr Yang graduated from the University of Technology Sydney and is an Australian citizen. ( Supplied )

Dr Yang has been held by China for more than 420 days over as yet unexplained allegations relating to espionage.

His detention has left him in poor health, according to Australian consular reports.

He was detained in the southern Chinese city of Guangzhou in January last year and formally arrested seven months later.

That formal arrest began a seven-month period of investigation.

Under Chinese law, investigators were required to either release or formally charge Dr Yang during that period, which expired on Monday.

This new step means Dr Yang is almost certain to now face trial in the Chinese judicial system.

Allegations are not clearly defined

To date, Beijing has not provided any information regarding what they accuse Dr Yang of doing, other than to say he is accused of "espionage".

On Monday ABC News revealed Dr Yang's background as an intelligence officer for China's most powerful spy agency, the Ministry of State Security (MSS), and how he eventually broke with the Chinese state to become a supporter of Western-style democracy.

Yuan Xiaoliang awaits news of her husband Yang Hengjun from their Beijing apartment. ( ABC News )

Dr Yang's wife, Yuan Xiaoliang, was also detained at Guangzhou Airport and subsequently interviewed by China's secret police.

She is concerned Beijing may seek to also charge her with endangering national security.

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Dr Yang's Beijing-based lawyer Mo Shaoping said he was informed by Chinese authorities on Tuesday of the decision to transfer Dr Yang's case to a legal supervisory body known as the People's Procuratorates, which in western terms would be similar to an office of government prosecutors.

Dr Yang's academic mentor, UTS professor Feng Chongyi (left), said the move to charge Dr Yang was outrageous. ( Supplied )

Under Chinese law, all cases that have been recommended for prosecution must be reviewed by the procuratorates.

Procuratorates examine the evidence required for the suspect to be to put on trial and, in most cases, issues a decision to prosecute within a month if it believes the facts of the crime are "clear".

They can also ask for further investigation or issue a decision to release the suspect.

The Chinese Government has prevented Australian consular officials from visiting Dr Yang since December, citing coronavirus infection concerns.

The lawyer has also been recently denied access to his client for the same reasons.

'No evidence to substantiate the charge of espionage'

Dr Yang's Australian friend and academic mentor, University of Technology Sydney professor Feng Chongyi, said he viewed the move as outrageous.

"There is no justifiable reason to transfer Yang's case to the prosecutors so they can approve a trial, because there is no evidence to substantiate the charge of espionage," Professor Feng said.

Dr Yang has repeatedly told people who have visited him in his Beijing detention centre that he is willing to prove his innocence in court.

Comment is being sought from the Chinese Government and Ms Payne.