Family and friends of detained Australian citizen Yang Hengjun say Chinese authorities look set to charge him with endangering state security.

Key points: Australian-Chinese Yang Hengjun was detained at Guangzhou in January

Australian-Chinese Yang Hengjun was detained at Guangzhou in January Authorities said he was suspected of endangering China's national security

Authorities said he was suspected of endangering China's national security His wife has been served with a formal notice of charges

Dr Yang, 54, has been detained in a Beijing state security prison for six months without access to his lawyers or family, but has recently been moved to a different facility.

His Chinese wife, Yuan Xiaoliang, travelled to Beijing and was served a formal notice of charges, one day ahead of a six-month deadline to either charge Dr Yang, release him or extend his detention.

Authorities said he was being investigated for endangering state security, but declined to comment on details of the case.

This week China's Foreign Ministry said Dr Yang was in good health and was being dealt with according to Chinese law.

His case is being handled by the Beijing branch of China's state security department.

If found guilty, Dr Yang could be sentenced to at least three years in prison.

Dr Yang detained during visit to China with wife and child

Yuan Xiaoliang and Yang Hengjun ( Supplied )

Yang Hengjun is a former employee of China's Ministry of Foreign Affairs who later gained Australian citizenship and became a prominent writer and outspoken online political commentator.

He was active on Chinese social media and Twitter, where he has more than 130,000 followers.

Dr Yang had been living with his family in New York, where he was a visiting scholar at Columbia University since 2017.

In January this year, he flew to the Chinese city of Guangzhou with his wife and child.

His family were allowed to board their connecting flight to Shanghai, but Dr Yang was escorted from the airport by authorities.

Later that month, the Chinese Foreign Ministry confirmed the arrest, stating that Dr Yang was suspected of "engaging in criminal acts that endangered China's national security".

His wife, who was born in China but is an Australian permanent resident, has been prevented from leaving the country.

Earlier this month, Ms Yuan was detained and questioned by the Ministry of State Security for two hours after she tried to fly out of China.

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