China says it is investigating a Chinese-Australian writer for alleged “involvement in criminal activities endangering China’s national security”.
Yang Hengjun’s detention in China was earlier disclosed by Australia – days after he went missing.
The 53-year-old former Chinese diplomat has been held since flying from New York to Guangzhou on Saturday.
Australian authorities have urged China to handle the matter “transparently and fairly”.
Foreign Minister Marise Payne said embassy officials would meet their Chinese counterparts on Thursday to “arrange consular access at the earliest possible opportunity”.
Mr Yang was being held in “residential surveillance” in Beijing, Australian officials said.
The term is often used when Chinese investigators hold a suspect at an undisclosed location. The interrogation process is thought to often include torture – a practice China denies takes place.
The Australian citizen had travelled to China with his wife, Chinese national Yuan Rui Juan, and young stepson.
It is believed that Ms Yuan has been interviewed by Chinese officials in Beijing, after first travelling to Shanghai to leave her son with relatives.
Who is Mr Yang?
He has a sizeable following online and has sometimes written critically about China’s Communist Party, but less so in recent times. He also authors spy novels.
Mr Yang was briefly unreachable on a trip to China in 2011 – prompting fears he was missing – but later attributed the episode to a “misunderstanding”.
He currently lives in New York and is a visiting scholar at Columbia University, said his friend Feng Chongyi, an academic at University of Technology Sydney.
Associate Prof Feng said that he had warned Mr Yang recently against travelling to China, but that Mr Yang had replied that he considered himself to be safe.
Mr Yang had not responded to messages in recent days, his friend said.
“He is one of the most influential bloggers on political issues in China, where he’s earned the nickname of ‘Democracy Peddler’ through talking about democracy and human rights,” Associate Prof Feng told the BBC on Wednesday.
What is the background to this case?
Correspondents say Mr Yang’s case follows a similar pattern to the cases of the two Canadian citizens, Michael Kovrig and Michael Spavor, who were detained in China in recent weeks.
In both cases, the foreign ministry initially said it had no knowledge of them being held, then confirmed they were in the hands of state security a few days later.
Australia has previously expressed “concern” about the arrests of the Canadians.
China has denied the detention of the two men is tied to Canada’s arrest of a senior Huawei official, Meng Wanzhou, but many analysts believe it is a tit-for-tat action.
Mr Yang’s detention comes as Australian Defence Minister Christopher Pyne landed in Beijing for official talks on Thursday.