Democracy activist Tran Hiep is jailed at B34 Detention Centre of the Ministry of Public Security in Ho Chi Minh City, according to consular correspondence sent to his wife Chloe Dang on Thursday.
"The correspondence (from Vietnamese authorities) notes that Hiep is in good health and is being detained in accordance with local regulations," the consular note seen by AAP said.
Mr Diep was due to meet his wife at Bangkok's airport after travelling to the Cambodian capital Phnom Penh to meet with human rights defenders.
But the 51-year-old Melbourne man never boarded his flight and has not been heard from since September 18.
Vietnamese authorities said on September 29 that Mr Hiep, also known as Ho Pham Hiep Huy, had been arrested along with two other activists but did not reveal their whereabouts.
Ms Dang, who has returned to Melbourne, said she was relieved to know his exact location but still worried for his safety.
"After nearly three weeks, I finally got a little more information about my husband," she told AAP hours after finding out his whereabouts.
"The Vietnamese government has stated he is in good condition, but I cannot stop worrying until he is able to meet with Australian consular officials.
"We still don't know when they can visit him."
Ms Dang lamented he had to spend his 51st birthday on Wednesday behind bars.
A United Nations human rights report described the decrepit conditions at the temporary detention facility, including windowless cells and a lack of oxygen with tiny slots in steel doors.
The Department of Foreign Affairs and Trade said it would not comment on individual cases because of privacy reasons.
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 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.
Viet Tan has accused Vietnamese authorities of kidnapping Mr Hiep and Mr Thuan.
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.
Vietnam has ramped up its crackdown on dissidents in recent years, with some 160 political prisoners locked up.
Activists face long stints of pre-trial detention, without access to lawyers or family in the one-party police state.
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