The lawyer for a Sydney retiree set to face trial in Vietnam for alleged "terrorist activities" has urged the Australian Government to work harder to help free his client, amid ongoing fears for his health.

Key points: Australian citizen Van Kham Chau is due to face trial for terrorism in Vietnam on November 11

Australian citizen Van Kham Chau is due to face trial for terrorism in Vietnam on November 11 Hanoi claims pro-democracy group Viet Tan is a terrorist organisation

Hanoi claims pro-democracy group Viet Tan is a terrorist organisation Mr Chau's lawyer is adamant the case is politically motivated

Australian citizen Van Kham Chau, 70, will face trial next week on November 11 in Ho Chi Minh City. He faces up to 15 years in prison if found guilty, according to Vietnamese law.

Mr Chau, who arrived in Australia as a Vietnamese refugee by boat in 1982, is a member of democracy and human rights group Viet Tan, which the Vietnamese Government has branded a "terrorist" organisation.

He is accused of entering the country on false documents and was meeting with a political activist during a human rights "fact-finding" mission when he was arrested in January this year.

Speaking to the ABC for the first time, his lawyer Phuc Vinh Trinh was adamant that Mr Chau is a political prisoner.

"They call him a terrorist because of that, but in reality, when he went to Vietnam he wasn't a terrorist — he didn't intend to commit a terrorist act," Mr Phuc said.

"All he did was write essays on Facebook calling for human rights in Vietnam. That's all."

A candlelight vigil was held for Mr Chau at Martin Place in Sydney in September. ( Supplied )

Mr Phuc said Mr Chau entered Vietnam through Cambodia on false papers, but said that was a "side issue".

Viet Tan's chairman Do Hoang Diem has previously said that "when facing such a lawless regime, dissidents and activists on their black list have no choice but to resort to finding other ways to enter Vietnam".

Mr Chau has been outspoken about human rights in Vietnam. ( Supplied )

"The main issue is the terrorism charges, for going against the Communist Government of Vietnam. That'll be the question for the courts," Mr Phuc said.

"There's a Viet Tan in Australia — are they a terrorist organisation there or not? If they're not, then you can't charge someone for being a terrorist by being a member of this organisation."

Mr Phuc said he would appeal for leniency for Mr Chau based on his age and ill-health, but in his case "there are legal avenues, or there's diplomacy".

"Our hope … is that he can go back to his home to Australia early, using diplomatic channels between Australia and Vietnam," he said.

Mr Chau's family, pictured in 1998, are concerned about his health in prison. ( Supplied: Chau Family )

"If the Australian Government were to raise the issue, with some force, using diplomacy, Vietnam could be more lenient."

Mr Chau's youngest son, Dennis Chau, told the ABC the ordeal had taken an emotional toll on his family.

"I'm worried for my dad's health over the course of the trial," he said.

"I'm probably more nervous now that we've got a date set and I'm sure my dad is feeling it even more. I wish we could be there with him."

The Department of Foreign Affairs and Trade (DFAT) said it was providing consular assistance to an Australian man detained in Vietnam, but said "for privacy reasons we are unable to provide further details".

Mr Chau's eldest son, Daniel, and his wife, Trang, are appealing for his release. ( ABC News: Mazoe Ford )

Consular assistance can involve prison visits, providing a list of lawyers, liaising with family members and attending court hearings, but consular staff cannot provide legal advice, intervene in legal cases or get Australians out of prison.

Australia's Minister for Foreign Affairs Marise Payne has been approached for comment.

Jailed for 10 months without trial

Mr Chau was arrested in mid-January during a meeting with an activist from the Brotherhood for Democracy group and has been held in a Vietnamese jail cell for more than 10 months without trial.

He was denied access to a lawyer for months to "protect the secrecy" of the investigation, Vietnamese authorities said, and has only met with Mr Phuc twice.

Mr Chau was initially investigated for breaching Article 109 of the Vietnamese Criminal Code for "operating to overthrow the people's government" — a charge that carries a death sentence or life imprisonment in the most serious cases.

But a consular note from DFAT, seen by the ABC, revealed that charge was later downgraded to Article 113 — "terrorism to oppose the people's government", which carries a maximum sentence of 15 years.

He will also be tried under Article 341, which refers to fabricating an organisation's seal or documents and carries a fine of 30 million to 100 million Vietnamese dong ($1,872 – $6,241), or between three months and three years in prison. It can carry a penalty of up to five years if the offence is committed by an organised group.

Social worker and Viet Tan member Hong Vo is anxious about Mr Chau's looming trial. ( ABC News: Erin Handley )

Melbourne social worker Hong Vo, a friend and fellow member of Viet Tan, said her group was concerned about the outcome of Mr Chau's case.

"Everybody is so anxious, because we don't know what's going on," she told the ABC.

"We don't know how much longer they are going to hold him."

Ms Vo was arrested and held in Hanoi for taking part in a protest in 2010, but was released after 10 days after Australian officials raised the issue with Vietnamese officials at a senior level.

Vietnam's Ministry of Foreign Affairs did not respond to the ABC's requests for comment by deadline.

As of May this year, at least 128 prisoners of conscience were detained in Vietnam, according to Amnesty International.