The wife of a Sydney retiree sentenced to 12 years in a Vietnamese prison has written an emotional letter to Prime Minister Scott Morrison, questioning whether the Government's "weak" response would have been stronger if her husband was a white Australian.

Key points: Sydney man Van Kham Chau was convicted and sentenced to 12 years in jail in less than five hours

Sydney man Van Kham Chau was convicted and sentenced to 12 years in jail in less than five hours He is due to appeal against his sentence but rights groups criticise Vietnam's 'opaque' courts

He is due to appeal against his sentence but rights groups criticise Vietnam's 'opaque' courts Mr Morrison wrote to Chau's wife, saying his Government "cannot intervene in another country's legal matters"

Australian citizen Van Kham Chau, a 70-year-old retired baker, was last Monday convicted and sentenced for "the crime of terrorism against the people's government" in a Ho Chi Minh City courtroom — he has lodged an appeal against the conviction.

The proceedings took less than five hours and Chau was cut off by the presiding judge as he made a statement in his defence.

Four days after Chau's conviction, Mr Morrison wrote to Chau's wife, Quynh Trang Truong, noting "you are likely disappointed with the outcome".

Sydney retiree Van Kham Chau, 70, has been held in a Vietnamese prison cell since January. ( Supplied )

"The Australian Government cannot intervene in another country's legal matters or court proceedings, or law-enforcement investigations. As with all arrest and prisoner matters overseas, Australians are subject to the laws of the country they are in," he wrote.

"The Government of Vietnam is aware of the interest in Mr Chau's case and his health and wellbeing. The Australian Government will continue to regularly engage with the Government of Vietnam on your husband's case.

"I trust this information will be of assistance to you."

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 dated November 20, Ms Truong thanked the Prime Minister for his response but asked him to do more for her husband, who suffers from prostate health problems.

"Whist I understand your diplomatic position, one would have to wonder if my husband were a white Australian that the intervention might not be so weak or reluctant to date," she wrote.

Chau Van Kham, pictured last year with his wife, Trang Chau, has lived in Australia since 1982. ( Supplied: Chau Family )

"Above all, having recently witnessed how you defied the odds and won the 2019 federal election, I continue to have faith in you, your leadership and capability to successfully save my 70-year-old husband from what has been largely perceived as equivalent to a death sentence for him.

"Being an elderly person myself, I have nothing else but unconditional love for my dear husband, and words to seek help for him.

"So please listen to my words of appeal one more time, if you still care for the current fragile life of an Australian citizen."

The Department of Foreign Affairs and Trade was approached for comment about the letter, but repeated a statement they made last week, saying: "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."

The ABC understands Mr Morrison has written a response to the Chau family.

What happened in the court room?

Chau was cut off by the judge as he made a statement in his defence, saying that he "loved the country". ( AP: Nhan Huu Sang/VNA )

Chau, who has been held for 11 months in Vietnamese prison since his arrest in mid-January, is a member of Viet Tan.

The UN's human rights office has described Viet Tan as "a peaceful organisation advocating for democratic reform", but the Vietnamese Communist Government brands it a "terrorist" group.

A man in a blue shirt, who appears to be Australian citizen Van Kham Chau, enters the Ho Chi Minh City court. ( Supplied )

Mr Chau told presiding judge Pham Luong Toan he would not have returned to Vietnam if he knew Vietnamese Criminal Law better, and that he joined Viet Tan because he "loves the country".

The judge then stopped Mr Chau and told him to return to his seat.

Last Monday, Chau was given 15 days to appeal against his conviction and sentence.

His lawyer at trial, Trinh Vinh Phuc, told the family his role ceased when the proceedings did and that Chau would have to file an appeal on his own.

Mr Phuc previously told the ABC diplomatic intervention from the Australian Government was Chau's best chance for release.

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

Dan Phuong Nguyen, an Australian lawyer representing the family, said the lack of legal representation in Vietnam's judicial system was cause for concern.

"I'm extremely concerned. We can't advise him," she told the ABC.

Members of the Vietnamese community place candles in the shape of Vietnam at a vigil for Mr Chau. ( ABC News: Erin Handley )

"It's totally unsatisfactory, and the Government is not saying anything or doing anything, or speaking out at all.

"We're appealing to the Government to help this 70-year-old who is needing medical attention, who has been unfairly trialled and who has got no help in his appeals process."

In a statement, she added that he had been "harshly sentenced" and added that the family had been asking for a meeting with Foreign Minister Marise Payne "with no response".

Vietnamese community upset by 'harsh' sentence

Dr Phong Nguyen, a Sydney-based cardiologist and Viet Tan member, said the case had rattled members of the Vietnamese diaspora in Australia.

"The whole Vietnamese community is quite upset of the harsh, unjust sentence from the Viet Government and disappointed with our Government's handling of Mr Chau's case," he said.

"We feel little support and intervention from our Government here."

Elaine Pearson from Human Rights Watch said the Australian Government's response was "pitifully weak" and that "the trial was a complete farce and should be called out as such".

"The whole idea of not 'interfering in the legal affairs of another country' is fine if the country respects rule of law and due process. But what if, like Vietnam, they don't?" Ms Pearson said.

Yang Hengjun has not seen his family since he was detained in Guangzhou, China, in January. ( Supplied: Twitter )

"Then it basically leaves Australian citizens hostage to governments with biased, opaque courts lacking fairness and due process."

Ms Pearson said the Government's response to the Chau case contrasted sharply with the approach taken on other Australian citizens Yang Hengjun and Hakeem al-Araibi.

"The question is, why? There doesn't seem to be the same level of public pressure regarding Van Kham Chau," she said.

"One has to ask, 'Why is one communist regime — Vietnam — being treated differently from another — China?'"

A rally has been planned in Canberra for next Monday, November 25, with Labor MP Chris Hayes set to give a press conference.

Mr Hayes, who claims to have one of the largest Vietnamese communities in the country in his electorate of Fowler, told the ABC he struggled to see any evidence presented to suggest Chau was a "terrorist".

Labor MP Chris Hayes said there was no evidence to suggest Chau was a "terrorist". ( AAP: Alan Porritt )

"Vietnam is a single-party state, and I have been critical that its judicial system does not appear to administer justice without fear or favour — it is clearly influenced by the whim and will of government," he said.

He added the Viet Tan members he met were often doctors, lawyers and social workers who were advocates for democracy and human rights, not terrorists.

He said he would be waiting to hear the result of the appeal and hoped Chau's sentence would be drastically reduced and he would be deported, as has happened in some other Vietnamese cases involving outspoken rights advocates.