Yang Hengjun, a blogger and former Chinese diplomat, has not been heard from since flying from New York to Guangzhou on Saturday, a friend said.
Sydney academic Feng Chongyi said he feared that Mr Yang had been detained by Chinese authorities.
Chinese officials have not yet commented.
In response to a query about Mr Yang, Australia’s Department of Foreign Affairs and Trade said it was “seeking information about an Australian citizen who has been reported missing in China”.
Mr Yang is believed to have been travelling with his wife, Chinese national Yuan Rui Juan, and young stepson.
A social media post by Ms Yuan appeared to show that she was in Beijing, Australian media reported.
It is believed that she was interviewed there by Chinese officials after first travelling to Shanghai to leave her son with relatives, reported the BBC’s Stephen McDonell in Beijing.
‘We’ve been trying to contact him’
Mr Yang, an Australian citizen who now lives in the US, has a sizeable following online and has been critical of China’s Communist Party. [FULL STORY]