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.
"I really worry about how he will be treated in the prison," Ms Dang told AAP on Tuesday, hours after landing in Melbourne from Bangkok.
"I really hope our government can help to get access to him as soon as possible."
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 admitted being members of Viet Tan, a pro-democracy diaspora political party founded by Vietnamese dissidents.
Viet Tan is a proscribed terrorist organisation under Vietnam's communist government.
The activists are accused of entering the country to conduct "terrorist and sabotage activities" as well as forging seals and documents.
Mr Hiep was carrying an Australian passport, according to the ministry.
He founded the Australian Alliance to Combat Human Trafficking and was travelling to meet with other Vietnamese activists in Cambodia, according to Viet Tan.
Viet Tan has condemned Vietnamese authorities for the arrests of Mr Hiep and Mr Thuan, and denies claims the two men illegally entered Vietnam from the Cambodian border.
The group claims the pair had no intention of entering Vietnam.
They accuse Vietnamese authorities of abducting the pair on trumped-up charges to cover up an act of "transnational repression".
Viet Tan's representative in Sydney, Phong Nguyen, said the arrests were a chilling deterrent to other members travelling to Southeast Asia, even for tourism reasons.
"We condemn this and it is unacceptable," he told AAP on Tuesday.
"The Vietnamese government has now one of the most severe records of repressing activists outside the borders of Vietnam."
Ms Dang called on Prime Minister Anthony Albanese to bring Mr Hiep home because of Vietnam's abuse of political prisoners.
"Please do and try anything to get him home safely to us - that's my wish right now," she said.
"My emotions are all over the place. My body is shaking, my heart is jumping and I now know he's in prison in Vietnam, but it worries me more because we don't know where he is exactly."
Vietnam has ramped up its crackdown on dissidents in recent years, with more than 55 activists arrested under broad criminal laws in 2025.
Foreign Minister Penny Wong's office said the Department of Foreign Affairs and Trade was making urgent inquiries with Vietnamese authorities to confirm "an Australian citizen has been detained."
Mr Albanese said the government was providing consular assistance and engaging with Mr Hiep's family.
"That's what Australian governments do for our citizens," he told reporters.
Sydney man Chau Van Kham, also a member of Viet Tan, was detained in 2019 in Vietnam and returned to Australia in 2023 after lobbying by the Albanese government.
With Reuters
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