For Trang Chau, her husband's beloved dragon fruit cacti, which he grows in the backyard of his family home in Sydney, is a constant reminder of him.

“He’s been growing dragon fruit for eight years," she tells SBS News.

"He puts soil and fertiliser to make the cactus grow and make the fruit sweet.”

Van Kham Chau fled Vietnam by boat and arrived in Australia in 1982, where he met his wife. The couple married four years later and had two sons, Daniel, 31 and Dennis, 29.

He opened a Sydney laundry shop and then a bakery, which he ran until his retirement five years ago, and has been an ardent democracy advocate since.

It was that commitment to the cause that led the 69-year-old to revisit his birth country on what a colleague described as a "fact-finding" mission before he was arrested on 15 January 2019.

Mr Chau is a member of pro-democracy group Viet Tan and it is believed he was in Vietnam doing research on the country's human rights record - something he continued to advocate for, as well as democracy, despite no longer living in the country.

Documents obtained by SBS News show one of the offences Mr Chau is being investigated for is attempting to overthrow the communist regime under article 109 of the Vietnamese criminal code, an offence that can lead to life imprisonment or even the death penalty in the most serious cases.

Mr Chau is also being investigated for allegedly breaching article 341, which relates to fabricating documents.

Vietnamese authorities allege he used a fake identity card to enter the country from Cambodia.

According to Professor Ben Kerkvliet at the Australian National University, who has researched pro-democracy activists in Vietnam since the mid-1950s, the article 109 charge is not often pressed by authorities.

"Only 11 per cent were charged with that kind of offence, most of them charged with spreading propaganda against the state which is a charge that is relatively small in consequence in comparison."

His family says the charges are farcical and simply could not be true.

“It’s ridiculous - my dad is a 69-year-old man pretty much by himself in Vietnam … It's trumped up vague charges to get something on him,” his son Dennis Chau told SBS News from London, where he now lives.

Mrs Chau has spoken publicly for the first time in the hope that the Australian government will lobby the Vietnamese government to expedite the case and push for Mr Chau's release.

“As a wife, no-one wants their husband to go into jail for that long or be (possibly) sentenced to death like that,” she said.

“He has a mother that is 95 years old. She misses him.

“Every day she asks when he will be back, but we don't know when. We hope the government will do more.”

No-one wants their husband to go into jail for that long or be (possibly) sentenced to death like that.

Bon Nguyen, the president of Australia's federal Vietnamese Community Association is also calling for greater government action.

“The government can definitely do more to help as he is a citizen of Australia,” he said.

“Australia and Vietnam entered into a strategic partnership in 2018 and we think Australia can be doing much more to expedite Mr Chau back to Australia.”

Reports from the Department of Foreign Affairs and Trade (DFAT) and seen by SBS News indicate Mr Chau is being detained under a four-month investigative period which can be extended at police discretion.

He is not permitted to have a lawyer and his family believes that he cannot speak freely during consul meetings due to the heavy presence of prison officials.

Human Rights Watch says its latest figures show at least 120 political prisoners are currently detained in Vietnam.

“Vietnam has an appalling human rights record particularly when it comes to civil and political rights,” its Australian director Elaine Pearson said.

“What we have seen in recent years is an increase in terms of the lengths of sentences of these prisoners, and also in terms of physical violent attacks against people participating in protests, and sometimes against activists themselves.”

The Vietnamese government has labelled Viet Tan a terrorist group - an assertion strongly refuted by its chairman, Do Hoang Diem.

“Viet Tan is a political party committed to peaceful change in Vietnam,” he says.

“We practice non-violent techniques to bring about social changes in Vietnam; we challenge the Vietnamese government to show proof that we are a terrorist organisation.”

Mr Do is based in the United States and will be coming to Australia in April to speak to parliamentarians and the Vietnamese people to raise awareness of Mr Chau's case.

Professor Ben Kerkvliet, a political scientist at ANU, says the Viet Tan has evolved in the last few decades.

"In the 70s, early 80s the Viet Tan wasn't a serious threat to the Vietnamese government ... there were efforts by the group to establish a military counter-insurgency but that didn't last too long.

"The Viet Tan then resorted to peaceful and long-term methods of bringing about change in the government."

In a statement provided to SBS News, DFAT said it would not comment on whether it is doing anything to expedite Mr Chau's case beyond providing consular assistance.

“The Department of Foreign affairs and Trade is providing consular assistance to an Australian man detained in Vietnam,” a spokesperson said.

“For privacy reasons we are unable to provide further details.”

By Lin Evlin - Special Broadcasting Service (.au) - March 10, 2019