Human rights groups and relatives have expressed concern for the wellbeing of a 70-year-old Australian citizen who has been imprisoned in Vietnam for six months without charge.

Van Kham Chau, who fled to Sydney from Vietnam in the 1980s, was arrested in Ho Chi Minh City in January for allegedly trying to overthrow the state and for entering the country on false documents.

He has been denied a lawyer during the investigation, his family and Amnesty International confirmed, meaning information on his case has been limited. However, the charges of treason for which he is being investigated carry prison sentences of up to 20 years, or even the death penalty.

Chau had been a vocal member of Việt Tân, an international pro-democracy group deemed by the Vietnamese government to be a terrorist organisation, and its members have regularly been detained when they step on to Vietnamese soil.

“Van Kham Chaus case shows the absurdity of Vietnam’s judicial system, where those charged with national security offences can’t even access a lawyer,” said a spokesperson for Amnesty International. “Everyone has a right to due process and adequate defence.”

According to his wife, Trang, and son Dennis Chau, Chau had been meeting a member of another prohibited pro-democracy group, Brotherhood for Democracy, when he was arrested in Ho Chi Minh City and taken to prison.

They have been unable to visit Chau in jail, and instead have had to deliver letters to him through the Australian consul in Vietnam. In a letter from jail in May, Chau said his “health is good, but my spirit is declining”, according to ABC.

His son described the allegations against Chau as “ludicrous”.

“He’s quite patriotic about his country, he just wants the best for the people there,” Dennis Chau told ABC.

His arrest comes against the backdrop of a broad crackdown by the Vietnamese government on people deemed to be critical of the regime. According to Amnesty International, the number of prisoners of conscience unjustly jailed across Vietnam has risen by one third to 128. In June, Nguyen Ngoc Anh, an environmental activist, was sentenced to six years in jail for writing critical posts on Facebook.

Foreign citizens have also been targeted. In June, the US citizen Michael Phuong Minh Nguyen was sentenced to 12 years in prison after a court found him guilty of attempting to overthrow the state. Last August, two Americans were jailed for 14 years for allegedly attempting to overthrow the state. Both were members of the California-based pro-democracy group Provisional National Government of Vietnam.