Tran Hiep, a democracy campaigner, was due to meet up with his wife, Chloe Dang, in Bangkok after travelling to Phnom Penh, Cambodia, 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.
Mr Hiep, who founded the Australian Alliance to Combat Human Trafficking, was travelling with Vietnamese-Norwegian human rights activist Nguyen Duc Thuan to meet with dissident Vietnamese activists for training in Cambodia.
Viet Tan, a pro-democracy diaspora political party, has accused Vietnamese authorities of kidnapping the pair 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, accusing 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 released on Monday night.
AAP has contacted Vietnamese authorities for comment.
Viet Tan is calling 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.
"My husband deserves protection, and I really hope the Australian government will continue to put pressure on Cambodia to do an investigation to find out where he is right now."
Vietnam has ramped up its crackdown on dissidents in recent years, with more than 55 activists arrested under broad criminal laws in 2025.
Civil society groups and United Nations human rights experts have repeatedly warned of growing co-operation between Vietnamese authorities and neighbouring governments targeting advocates.
"Their disappearance is not an ordinary 'missing person' incident. Both men are long-time human rights advocates and Viet Tan members," Viet Tan said.
Ms Dang, 36, 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 confirmed consular officials were in contact with Ms Dang but declined to comment further.
"The Department of Foreign Affairs and Trade is providing consular assistance to the family of an Australian reported missing in Cambodia," a department spokesperson told AAP on Monday.
The families of the missing men have filed an appeal with the UN Working Group on Enforced or Involuntary Disappearances.
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