Two lawyers appointed by the wife of an Australian citizen detained in China have been denied access to him, with Chinese authorities claiming it is at the detainee's request.

Key points: Yang Hengjun is a former Chinese Foreign Ministry employee

Yang Hengjun is a former Chinese Foreign Ministry employee He had been living in the US when he was arrested on arrival in Guangzhou last month

He had been living in the US when he was arrested on arrival in Guangzhou last month He is being held under 'residential surveillance' somewhere in Beijing

Yang Hengjun, 53, is being held in an undisclosed location in Beijing under investigation for 'endangering state security', on charges that may involve espionage.

One of two lawyers appointed by Mr Yang's wife to take on the highly sensitive case said he was informed by State Security authorities handling the case that Dr Yang would not accept the lawyers appointed by his family.

"They also denied us access to talk to Mr Yang face to face to confirm this", Shang Baojun said.

Yang Hengjun, whose legal name is Yang Jun, is a former Chinese Foreign Ministry employee who later migrated to Australia and became a prominent online writer.

He had been living in the US with his second wife and her family but travelled to the southern Chinese city of Guangzhou last month and was detained upon arrival.

He has two sons living in Sydney.

Dr Yang is being held at an undisclosed location under 'residential surveillance' — a secretive form of detention that allows investigators to interrogate him for up to six months without laying charges or allowing visits from family or lawyers.

Australian consular officials were however allowed to visit him six days after he was first detained.

"If this is Mr Yang's choice or if he's appointed other lawyers, of course we'll respect that, but I hope next time Australian consular officials visit him, they'll have a chance to verify this with him", Mr Shang said.

Yang Hengjun is a former employee of China's Foreign Ministry. ( Supplied: Twitter )

Shang Baojun's colleague Mo Shaoping was also appointed by Dr Yang's wife Yang Ruijuan, and knows Yang Hengjun well.

When confirming Yang Hengjun's arrest in late January, China's Foreign Ministry spokeswoman Hua Chunying said his legitimate rights and interests were fully guaranteed, but authorities have not publicly disclosed the reasons for the investigation.

The Australian Government has called on China to treat Yang Hengjun "fairly and transparently".