"The Chinese authorities had not informed the Australian government until the media reports," Dr Feng said.
"If there's no media report, there's nothing to lead to the government inquiring."
China accused Dr Yang of "endangering national security".
He was held without charge for almost eight months — until now.
Dr Yang was detained in what China calls "residential surveillance" at an undisclosed location in Beijing from January until July.
"We would describe it as home detention. As Dr Yang doesn't have a home in Beijing, he is being held in a similar situation as opposed to being held in a prison," then Defence Minister Christopher Pyne said in January.
Human rights organisations — and Dr Feng — believe China's so-called "residential surveillance" system is more sinister than that.
"The authorities can detain someone for as long as six months without laying a formal charge. When they do not have enough evidence, they do that," Dr Feng said.
He said Dr Yang could endure "round the clock interrogation" in an attempt to force a confession.
On July 19, Australia received formal notification from Chinese authorities that Dr Yang had been moved to criminal detention.
Australian consular officials met with Dr Yang multiple times in the months following his initial arrest, and the Department of Foreign Affairs and Trade (DFAT) expressed concern that he had not been given access to proper legal representation.
Foreign Minister Marise Payne said the government had raised its concerns about the case "regularly" with China at "senior levels". Ms Payne also wrote to her Chinese counterpart to request "a fair and transparent resolution", and to demand Dr Yang be given access to his lawyers.
"If he is being detained for his political views, then he should be released," the Foreign Minister said in July.
"We have worked tirelessly and in good faith with the Chinese government to advocate for Dr Yang's interests since he was detained. We expect basic standards of justice and procedural fairness to be met."
Meanwhile Dr Yang's supporters, Dr Feng foremost among them, accused the Australian government of a "very, very weak" response.
"The Australian government has an obligation to ask for the immediate release of their citizen rather than saying in these very soft words that it's a police matter under Chinese law," Dr Feng said.
He said there should "absolutely" have been a stronger push for Dr Yang's release, more along the lines of the successful international campaign to free Australian football player Hakeem Al-Araibi from prison in Thailand.
Dr Feng believed the government's subdued treatment of China could put other Australians at risk in the future.
"If you allow this to become a precedent, then the authorities can simply detain anyone for any political consideration," he said.
The organisation Reporters Without Borders says there are more than 60 journalists and writers behind bars in China, including the award-winning photographer Lu Guang.
In its 2018 World Press Freedom Index, China ranked 176th out of 180 countries.