A Sydney retiree has been found guilty of "terrorist activities" and sentenced to 12 years in prison by a Vietnamese court, in what rights groups have called a "sham trial".

Key points: Van Kham Chau was convicted for holding a senior position in Viet Tan, a democracy and human rights organisation

Van Kham Chau was convicted for holding a senior position in Viet Tan, a democracy and human rights organisation Vietnam considers the pro-democracy group a "terrorist" organisation

Vietnam considers the pro-democracy group a "terrorist" organisation Mr Chau faced a judge-only trial in Ho Chi Minh City where only journalists from chosen media organisations were allowed inside

Van Kham Chau, 70, who is a retired baker from Sydney, is a member of democracy and human rights organisation, Viet Tan, which is considered a "terrorist organisation" by the Vietnamese Government.

Chau was convicted on Monday for holding a senior position in the New South Wales chapter of Viet Tan, for recruiting new members and for "financing terrorism".

Chau's youngest son, Dennis, told the ABC he was in shock.

"That's the worst possible news," he said.

Chau has the right to appeal within 15 days. After the appeal, a final sentence will be handed down.

"A 12-year prison sentence in a Vietnamese prison, where the conditions are horrible, is the equivalent to a death sentence," Phil Robertson, deputy Asia director for Human Rights Watch told the ABC, adding that the appeal "gives Australia the opportunity to stand up and get that sentence reduced."

His wife and two sons have made repeated appeals to the Australian Government to speak out against his 10-month detention, while his lawyer is adamant that Chau is a political prisoner.

"The Australian Government really has to step up and defend their citizen. They've been remarkably quiet and I don't understand why," Mr Robertson said.

"[Monday]'s verdict calls into question communist Vietnam's legitimacy and whether Australia, the United States or any law-abiding nation can forge a sustainable economic or security partnership with Hanoi," Duy Hoang, a spokesman for Viet Tan told the ABC.

"When the Vietnamese Government considers peaceful human rights activists as 'terrorists' how can any responsible country engage in security cooperation with Hanoi?"

According to Chau's lawyer Trinh Vinh Phuc, diplomatic intervention from Australia was the strongest chance in bringing Chau back to Australia "very early".

"If the Australian government raises a strong voice [on] this case then [we] will have more chance to change the situation," he told the ABC.

Accused of 'financing terrorism'

Chau arrived in Australia by boat as a Vietnamese refugee in 1982 and raised two children with his wife, Trang, in Sydney.

He was arrested in Ho Chi Minh City in mid-January while meeting an activist from the Brotherhood for Democracy group.

For months he was denied access to a lawyer to "protect the secrecy" of the investigation and later charged with breaching Article 113 — which means he has engaged in "terrorist activities" to oppose the Government.

Chau, along with his co-accused Nguyen Van Vien and Tran Van Quyen, faced a judge-only trial in Ho Chi Minh People's Court on Monday amid tight security, where only media from chosen organisations were allowed inside the courtroom.

The ABC understands many family members of the accused hoping to witness the trial were denied entry.

"It has been very difficult for his family members to get the paperwork to gain access into the trial because there are so many layers of security and because you have such a short time to prepare [the paperwork]," said Mr Phuc.

"With cases like this, they rarely allow family members to have contact with the accused."

The court was given a 21-page indictment, which described Chau's dealings with the organisation, detailing how he holds a senior position in Viet Tan as secretary of the Sydney chapter.

It also said he is accused of spreading Viet Tan's message, using false documents to come through Cambodia to Vietnam and providing financial support, which has been labelled as financing terrorism.

He is also accused of recruiting new members, giving instructions and setting up a Facebook presence to answer questions about the organisation.

'Please help': Wife appeals to PM

The Chau family has made repeated appeals to the Australian Government to speak out about Van Kham Chau's detention. ( ABC News: Mazoe Ford )

In a letter to Prime Minister Scott Morrison before the verdict, Trang Chau said her husband had been imprisoned for almost a year "as a result of his peaceful human rights activism".

Chau Kham Chau came to Australia as a refugee in 1982 and raised a family in Sydney. ( Supplied: Chau Family )

"He has also been hospitalised twice since detainment in Vietnam due to critically deteriorating health conditions," she wrote.

"In January 2019, right after my husband was detained in Vietnam, I wrote a letter to you with very strong hope that with empathy and compassion, you would help to save him.

"Now I am writing this second letter, filled with both faint hopes and great despair, to seek your valuable help again so my husband can be released and reunited with the family in Australia.

"He is a hard-working citizen, and he deserves the Australian Government's support in this painful circumstance.

"Please help, Prime Minister, as your assistance and empathy are strongly needed right now, to which my family and I will forever be indebted."

Vigils calling for Chau's release were held in Sydney in September. ( Supplied )

The Prime Minister's Office told the ABC it was a matter for the Minister of Foreign Affairs.

Mr Morrison declined to comment on the case during a visit to Vietnam last week.

"I've said this on many occasions, I was saying it just last week — Australians need to abide by the laws of the countries which they visit. They must," he said, according to the Sydney Morning Herald.

"They don't get a leave pass from laws when they go into someone else's country and commit crimes.

"That is not something that Australia can support or excuse. But we will always seek to support our citizens in these difficult circumstances."

'His guilt will be pre-determined'

The Minister for Foreign Affairs, Marise Payne, told the ABC: "As Mr Chau is able to appeal the sentence under Vietnamese law, it would be inappropriate, and not in Mr Chau's best interests, to comment while legal processes remain available.

"However, the Government of Vietnam is well aware of our interest in Mr Chau's case and his welfare.

"We will continue to engage with our Vietnamese counterparts on this matter."

The Department of Foreign Affairs has also been contacted for comment.

The Vietnamese Government is against the Viet Tan organisation, of which Chau was a member. ( Reuters: Nguyen Huy Kham )

"The Vietnamese court system is controlled lock, stock and barrel by the Communist Party in Vietnam, this is a Kangaroo Court," Mr Robertson of Human Rights Watch told the ABC.

"They are literally show trials for the purposes of saying that there was a trial when in reality, the decision has already been taken about how they're going to treat the case."

"The reality is this is a 70-year-old man being charged with terrorism. There's no evidence that he was involved in anything violent whatsoever," he said.

Van Kham Chau was accused by Vietnam of being a member of a "terrorist organisation". ( Supplied )

Mr Robertson added that Hanoi most feared bad publicity on the world stage, and that "by staying quiet … Australia is playing into the hands of Vietnam."

Viet Tan chairman Do Hoang Diem condemned the "sham trial", saying Chau was a dedicated human rights activist and prominent member of the Vietnamese community in Sydney.

He said Chau "entered Vietnam to gain first-hand insight into the human rights situation in the country".

"We challenge the Vietnamese Government to provide any form of evidence linking them to 'terrorism'," Mr Diem said.