The Federal Government has issued its strongest statement yet on the case of Australian citizen Yang Hengjun, declaring it is "deeply disappointed" the academic has been transferred to criminal detention in China.

Key points: The deadline for authorities to either charge or release Yang Hengjun is approaching

If found guilty, he could be sentenced to three years in jail

Australia's Foreign Affairs Minister said the Government has expressed concern about his welfare and the conditions he is living in

Sydney-based writer and political commentator Yang Hengjun has been detained for six months in a residential surveillance centre in southern Beijing.

He has now been moved from a state security prison, officially dubbed "residential surveillance at a designated location" to a criminal detention centre, just ahead of a deadline for authorities to either charge or release him.

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

In a statement, Foreign Affairs Minister Marise Payne said the Government had received formal notification from Chinese authorities that the 54-year-old had been moved on Friday.

"The Australian Government is deeply disappointed that Australian citizen and academic Dr Yang Hengjun has been transferred to criminal detention in China.

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"The Government has expressed concern about Dr Yang's welfare and the conditions under which he is held and we have asked for clarification regarding the reasons for Dr Yang's detention," she said.

"If he is being detained for his political views, then he should be released."

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.

His lawyer, Shang Baojun, told AM it could be weeks before Dr Yang was formally charged.

"It's not certain they'll announce charges.

"The criminal detention order he's now under is still part of an ongoing National Security investigation.

"After this detention is finished, which could be at least two weeks, then it will be up to prosecutors to decide whether to charge him."

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

Even Dr Yang's lawyer does not know what he is alleged to have done

Yang Hengjun once worked for China's government, either within the Foreign Ministry or possibly for a branch of the State security ministry.

He later moved to Australia, where he started writing online and became a voice critical of China's government.

But in more recent years, he has been seen as a moderate, even appearing at a state media function in Beijing.

The lack of answers on what he has alleged to have done has baffled friends and family.

Yang Hengjun with his wife, Xiaoliang Yuan, who has been prevented from leaving China. ( Supplied )

Even his lawyer, Shang Baojun, does not know.

"Because he's accused of 'endangering national security', it's impossible for his family members to visit him.

"His lawyers can apply to meet him, but that's decided by the relevant authority."

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, saying 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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