Yang Hengjun has been in Chinese custody since he arrived in southern China’s Guangzhou from New York on 19 January with his wife, Yuan Xiaoliang, and his 14-year-old stepdaughter.

An Australian academic arrested in China accused of being a spy has asked Prime Minister Scott Morrison to help him “go home as soon as possible”.

Yang Jun, an outspoken writer who also goes by his pen name Yang Hengjun, has been detained in China since January without access to his lawyer or family.

Beijing announced his formal arrest last Friday on spying charges, which could bring a lengthy prison sentence.

Yang on Thursday pleaded for Morrison to “help me go home as soon as possible” and expressed his gratitude to Australian officials for their work on his case, in a statement released via a consular official.

A Chinese investigator “told me that Australia was small and wouldn’t care about me”, Yang said in the letter, obtained by national broadcaster ABC.

“He said Australia was dependent on China for its trade and economy, and Canberra wouldn’t help me, let alone rescue me. He said Australia wouldn’t help because I am not white.

Ties between the two sides have markedly deteriorated in recent months and China’s near-silence about Yang’s fate has only further strained relations.

Morrison said Thursday the suggestion that Yang acted as a spy for Australia was “absolutely untrue”.

“We’ll stand up for our citizen and we’ll expect him to be treated appropriately and his human rights to be respected,” he said in a televised interview.

Morrison also pushed back against Beijing’s warning to Australia that it “must not intervene in any way to China’s handling of the case”.

“We’ll continue to raise concerns as is appropriate,” he said. “We would expect the same to occur if other countries had concerns about anyone’s treatment in Australia.”

Yang is the latest in a string of foreign nationals to be arrested in China and charged with espionage or attempting to steal state secrets.

Two Canadians, former diplomat Michael Kovrig and businessman Michael Spavor, were detained in December.

Australia has traditionally been keen to avoid friction with Beijing, but Yang’s arrest will increase public pressure on Canberra to take a tougher line against its most important trade partner.

Beijing’s influence in Australian politics, increasingly aggressive foreign policy and growing military clout in the Pacific have sparked concern in Canberra.