Nguyen Duc Thuan (left) and Tran Hiep have been detained in Vietnam in what a pro-democracy group has called an act of "transnational repression"

A pro-democracy group has accused the Vietnamese government of abducting two of its members, an Australian and a Norwegian citizen, in Cambodia and taking them back across the border.

Vietnamese authorities confirmed that they had arrested Tran Hiep and Nguyen Duc Thuan, but say they had entered the country using forged documents and planned to carry out terrorist activities.

It said the men had admitted to being members of Viet Tan, which Vietnam sees as a terrorist organisation.

Viet Tan called their arrest a "blatant fabrication intended to conceal the fact that the Viet Tan members were abducted in Cambodia and forcibly taken to Vietnam".

The group, which has members in Vietnam and abroad, said the two men had been travelling in the southern border district of Kaoh Andaet in Cambodia when they went missing and had not been heard from since 18 September.

Tran's wife, Chloe Dang, told Australian media he had been meeting pro-democracy activists in Cambodia and had no intention of travelling to Vietnam.

He was meant to meet her in Bangkok to board a return flight to Australia but never turned up, she said.

"He always knew there was a risk doing human rights work, but we never thought that this would happen," she told the ABC.

According to a Vietnamese state media report published on Monday, Tran and Nguyen were detained with a third man at the border in Tay Ninh province on 18 September.

The trio "confessed to being members of the terrorist organisation 'Viet Tan,' [and] carrying forged documents to illegally enter Vietnam in order to carry out terrorist and sabotage activities," the report said.

They were charged on Monday with membership of a terrorist organisation while an investigation continued "to clarify the criminal acts of the suspects in order to handle them according to the law", it said.

"A state orchestrating the abduction of individuals abroad and forcibly taking them to its territory is a serious act of transnational repression," said Viet Tan. "It violates Cambodia's sovereignty and the fundamental principles of international law."

The Viet Tan statement did not mention the third man, who was named by authorities as Thai Cong Truong Son.

Australian Prime Minister Anthony Albanese said consular support was being provided to Tran's family.

"That's what Australian governments do for our citizens. And we'll continue to engage directly with the family."

"Vietnam authorities' confirmation that they detained [these] activists... demonstrates yet again that Hanoi is arguably the most vicious and pernicious practitioner of transnational repression today in Southeast Asia," Phil Robertson, Director of Asia Human Rights and Labor Advocates said in a statement.

The Vietnamese and Cambodian authorities have been contacted for comment.