The former Chinese government official turned democracy advocate released a message through his supporters after being visited in detention by embassy officials.

"I am deeply indebted to the Australian embassy for coming to see me," Dr Yang said.

He said a Chinese state security investigation officer told him Australia was small and would not care about him.

"He said Australia was dependent on China for its trade and economy, and Canberra wouldn't help me, let alone rescue me," Dr Yang said.

"He said Australia wouldn't help because I am not white.

"This is nonsense. He was wrong. I am extremely grateful to the Australian prime minister, foreign minister and members of parliament, the embassy team, and the ambassador for their help."

Mr Morrison said China's suggestions Dr Yang was a spy were untrue.

The prime minister also denied Australia was interfering in his case.

China warned Australia to respect its sovereignty and butt out of the case, but Mr Morrison said the government is concerned about Dr Yang.

"We make no apologies for standing up for one of our citizens," he told Nine's Today program on Thursday.

"There's their own justice process that they'll follow in China and that's appropriate, but these suggestions that he's acted for a spy for Australia are absolutely untrue."

Mr Morrison said he respects China's sovereignty.

"But we do expect Australians, indeed all citizens, to have their human rights appropriately looked after."

Foreign Minister Marise Payne has reminded China it must comply with international laws banning torture, inhumane treatment and arbitrary detention.

Victorian barrister Julian McMahon - who helped try to save two Australian members of the Bali Nine from the death penalty - recently joined the case alongside Melbourne lawyer Rob Stary.

Mr McMahon said it could be months before details of the broad spying allegations are known.

Unlike the Australian system, people in China are detained, arrested and charged in three separate steps.

"The problem we have at the moment is that although our client has been detained, we don't have material, we don't have the basis of the detention except a vague allegation," he said.

"And we don't have the timetable."

But Mr McMahon understands Dr Yang is in good spirits.

Labor foreign affairs spokeswoman Penny Wong said governments were entitled to advocate for their own citizens.

"The reasons for his detention should be clarified and we have said that if he's been detained for political reasons he should be released."

Dr Yang is a former Chinese diplomat who went on to become a pro-democracy campaigner and was made an Australian citizen in 2002.

The 54-year-old was detained in Guangzhou in January after flying into the country from New York where he was living with his family and working as a visiting scholar at Columbia University.

Australian Associated Press