Democracy campaigner Tran Hiep was due to meet his wife, Chloe Dang, at Bangkok's airport after travelling to the Cambodian capital Phnom Penh to meet with human rights defenders.
But the 50-year-old Melbourne man never boarded his flight and has not been heard from since September 18.
Vietnamese authorities confirmed late on Monday that Mr Hiep, also known as Ho Pham Hiep Huy, had been arrested along with Vietnamese-Norwegian human rights activist Nguyen Duc Thuan and Thai Cong Truong Son.
The country's Ministry of Public Security said the trio were detained on September 18 while allegedly attempting to cross illegally from Cambodia into Vietnam.
It said they had admitted to being members of Viet Tan, a pro-democracy diaspora political party which is a proscribed terrorist organisation under Vietnam's communist government.
The men are accused of entering the country to conduct "terrorist and sabotage activities" and of forging seals and documents.Â
Ms Dang she was proud of her husband and was upset he had been labelled a terrorist.
"Viet Tan, which he is a member of, is only to raise awareness about human rights and freedom of speech, so that has nothing to do with terrorism," she told AAP.
"He's been a human rights activist long before we met each other."
The 36-year-old said her husband led a regular life working, fixing things around their home at St Albans in Melbourne's west, and fishing and camping with her on weekends.
Viet Tan has accused Vietnamese authorities of kidnapping Mr Hiep and Mr Thuan.
The group denies claims the two men illegally entered Vietnam from the Cambodian border.
"This is a blatant fabrication intended to conceal the fact that the Viet Tan members were abducted in Cambodia and forcibly taken to Vietnam," the group said in a statement.
Ms Dang says her husband had no intention of going to Vietnam and claimed the Vietnamese government's timeline of Mr Hiep's alleged movements did not add up.
She said he was due to board his flight from Cambodia to Bangkok just one or two hours after the time authorities claim he tried to illegally cross the border into Vietnam.
"I (was) going to be the one to pick him up at the airport," she told ABC Radio.
"It's impossible."
Vietnamese authorities did not inform the Australian embassy in Hanoi or the federal government of her husband's arrest, Ms Dang said, and she found out through the news.
She still doesn't know where in Vietnam her husband is being detained.
"As a wife, I worry about that as well and sometimes I used to tell him 'always be careful because outside of Australia, it's different and you don't know what's going to happen'," Ms Dang said.
Mr Hiep founded the Australian Alliance to Combat Human Trafficking and was travelling to meet with dissident Vietnamese activists for training in Cambodia, according to Viet Tan.
Viet Tan has called on Norwegian and Australian authorities, as well as human rights organisations, to press the Vietnamese government for the immediate and unconditional release of both men.
Vietnam has ramped up its crackdown on dissidents in recent years, with more than 55 activists arrested under broad criminal laws in 2025.
Prime Minister Anthony Albanese on Tuesday said the government was providing consular assistance and engaging with Mr Hiep's family.
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