Democracy campaigner Tran Hiep was due to meet his wife, Chloe Dang, in Bangkok 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.
Vietnam'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, which is a proscribed terrorist organisation under Vietnam's communist government.
The group is accused of entering the country to conduct "terrorist and sabotage activities" and of forging seals and documents.
Mr Hiep was carrying an Australian passport at the time of his detention, according to the ministry.
He 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, a pro-democracy diaspora political party.
Viet Tan has accused Vietnamese authorities of kidnapping Mr Hiep and Mr Thuan, and deny claims the two men had illegally entered Vietnam from the Cambodian border.
The democracy advocacy group, of which Mr Hiep and Mr Thuan are members, claims the pair had no intention of entering Vietnam.
They accuse Vietnamese authorities of abducting the pair on fabricated charges to "cover up an act of transnational repression".
"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.
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.
"Mr Thuan is diabetic and requires daily insulin injections. He must be guaranteed full access to medication and medical care," the organisation said.
Ms Dang said she feared for her husband's life.
"My family and I are really worried about his safety, and we just only want to know where he is and what happened to him," she told AAP from Bangkok airport before boarding a flight to Melbourne.
Vietnam has ramped up its crackdown on dissidents in recent years, with more than 55 activists arrested under broad criminal laws in 2025.
Ms Dang said her husband's disappearance had taken a psychological toll.
"It's been 11 days and I'm really in the dark," she said, choking back tears.
"I don't know where he is and how he's doing - it's driving me crazy.
"Everything is really uncertain."
The Australian government earlier confirmed consular officials were in contact with Ms Dang but declined to comment further.
Prime Minister Anthony Albanese on Tuesday said the government was providing consular assistance and engaging with Mr Hiep's family.
"That's what Australian governments do for our citizens," he said.
With Reuters
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