A prominent Australian human rights advocate remains behind bars in Vietnam for a crime that carries the death penalty.

Long-time Sydney resident Chau Van Kham, 69, was arrested in Ho Chi Minh City last month while meeting with a civil society activist on a human rights fact-finding mission to his former homeland.

The retired small business owner is being detained for alleged activities against Vietnam's communist government.

Australian human rights advocate Chau Van Kham (pictured) is behind bars in Vietnam

Vietnamese authorities also allege he used a fake identity card to enter the country, the ABC reported.

His family are devastated and revealed that Mr Chau has been denied access to legal representation.

[My mum] always tries to be strong … but I can tell how she's breaking down,' his son Dennis told the ABC.

Mr Chau was feared missing for two weeks after he stopped responded to messages before the Vietnamese government eventually confirmed he had been detained.

Mr Chau's wife Trang and their son Dennis and Daniel are concerned about him

Mr Chau's family is also concerned about his health.

'If he does eventually come home, is he going to be the same? Or is he going to be frail or broken? I just don't know,' Dennis said.

A Vietnamese refugee, Mr Chau arrived to Australia's shores by boat in 1982 and is an active member of the pro-democracy group Viet Tan.

The organisation took to Instagram on Saturday to post a photo of two Sydney police officers showing their support for the campaign for Mr Chau's release.

Mr Chau's family suspected he was on a government watch list for his pro-democracy activities, which they used to joke about.

Dennis told the ABC that his father continues to campaign for those in his former homeland because they don't have basic freedoms.

A Change.org. petition calls on Prime Minister Scott Morrison and Foreign Affairs minister Marise Payne to intervene and demand for Mr Kham's immediate release

'I think that motivates him to do it. He's always been very, very passionate about it,' Dennis said.

'He's always put other people ahead of himself.'

The Department of Foreign Affairs and Trade confirmed it's offering consular assistance to an Australian man detained in Vietnam but wouldn't provide further details due to privacy reasons.

Mr Chau (pictured with his wife) isn't allowed to speak to his family or have access to a lawyer

A Change.org. petition signed by more than 2500 people calls on Prime Minister Scott Morrison and Foreign Affairs minister Marise Payne to intervene and demand the Hanoi government for Mr Kham's immediate release.

'A well known member of the Vietnamese community in Australia, Chau Van Kham has worked to shed light on the Hanoi's government's ongoing crackdown of free expression and peaceful activism,' the petition states.

'According to his family, Chau Van Kham received his first consular visit after more than two weeks and is being denied legal representation by the Vietnamese authorities.'

Mr Chau being detained for alleged activities against Vietnam's communist government

The petition also calls for Mr Chau to be reunited with his family.

'We believe that the Australia-Vietnam Comprehensive Partnership can only be realised when the Hanoi government respects basic human rights and treats everyone fairly,' the petition states.

Viet Tan and Mr Chau's family are in close contact with the Australian foreign ministry.

'We are mindful that the Vietnamese police have a history of framing peaceful activists with fabricated charges,' Viet Tan spokesman Phong Nguyen recently told AAP.