An Australian-Chinese writer detained on espionage charges left a letter written calling for a more democratic China with instructions to release it if he was ever arrested.

Yang Hengjun, who was detained on January 19, is being held in Beijing on suspicion of endangering China's national security.

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The letter, written in 2011, has been given to the media by his close friend, Sydney academic Dr Feng Chongyi after it became apparent Mr Yang would not be released anytime soon.

In the letter, Mr Yang urges activists to "maintain belief in China's democratic future, and, when it doesn't put yourself or your family at risk, to use all your means to push China's democratic development to happen sooner".

He implores his supporters to push for "freedom, human rights, the rule of law, and justice to occur sooner" in China.

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"If I can come out, I will continue my work. If I cannot come out or disappear again, remember my articles and let your children read them," he wrote.

He also apologises for not being honest about his earlier arrest in China in 2011, explaining that it was not due to a "misunderstanding".

He said he was, in reality, abducted by government agents.

"If I came out and said 'the truth', the authorities concerned would have been intolerable for a while, however, I can no longer return to China anyway, and can no longer continue to spread the tenants of freedom and democracy - which I was already doing with some scope," he wrote.

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"I really believe that in China, spreading the philosophy [of democracy] is a vital task.

"I will definitely continue to use my methods to contribute to China's democracy."

Mr Yang has been an Australian citizen since 2002.

The 53-year-old had been living in New York as a visiting scholar at Columbia University, before leaving for Guangzhou on January 18.

He was prevented from boarding his connecting flight to Shanghai with his wife and daughter and is said to be under residential surveillance in Beijing, which is similar to home detention.