Radio host Bin Lin has good reason to fear that he will never be able to visit the family he left behind in his home country if Hong Kong's extradition bill is passed.

The outspoken Communist Party critic is still appalled by the day he arrived at his China Radio Network office in Ultimo in 2010 to find a hand-delivered envelope addressed to him.

Journalist and language teacher Bin Lin is concerned about Hong Kong's proposed extradition laws. Credit:James Brickwood

Inside was a letter written in Chinese characters warning him not to mention the 1989 Beijing massacre again and reminding him of the 1967 assassination of Hong Kong radio host Lam Bun. "Next time it will be you," it said.

Dr Lin migrated to Australia from Hong Kong in 1991 in anticipation of the colony's transfer to China, which he feared would strip back the human rights that citizens had enjoyed under British rule.