Hong Kong: A prominent Australian democracy advocate has been detained by Vietnamese security police in Ho Chi Minh City and has not been heard from in almost two weeks, the pro-democracy group Viet Tan says.

Australian citizen Chau Van Kham has reportedly been detained in Vietnam. Credit:Viet Tan

Chau Van Kham, a member of Viet Tan, was detained after he crossed the land border from Cambodia on a fact-finding trip in his native country in early January and travelled to HCMC where he met with Nguyen Van Vien, a member of the Brotherhood for Democracy.

"We are mindful that the Vietnamese police have a history of framing peaceful activists with fabricated charges," Phong Nguyen, a Viet Tan spokesman told AAP. "At the moment we still haven't heard anything."

"His family are very concerned because of his ill health. He needs regular medication because of his prostate and we're trying to send medicine but we need to locate where he is first."