Australia secretly held a group of 46 Vietnamese asylum seekers on a warship at sea for almost a month and rejected their refugee claims during interviews that took as little as 40 minutes before returning them all to Vietnam last month, officials said.

The Vietnamese men, women and children were intercepted by an Australian border security vessel on March 20 and the asylum seekers were returned to the Vietnamese port of Vung Tau on April 18, Immigration Department secretary Michael Pezzullo told a Senate committee late Monday.

Maj. Gen. Andrew Bottrell, the army officer who oversees Australia's border security operations, said their refugee claims were rejected after individual interviews at sea that lasted between 40 minutes and 2 hours.

Pezzullo said the interviews were long enough to ensure that none of the asylum seekers' claims met Australia's protection obligations under the United Nations Refugee Convention.

The Vietnamese were returned to their homeland after Vietnamese officials gave assurances they would not be punished, Pezzullo said.

"There was a level of assurance provided that there would not be any retribution for their illegal departure from Vietnam," he said.

Bottrell said the written assurances from Vietnam that provided a "level of comfort" about sending the asylum seekers back, although Australia had not kept track of them since their return.

United Nations refugee agency last month raised concerns at reports that the Vietnamese refugee applications had been screened at sea, questioning whether they had been properly judged. An UNHCR official was not immediately available for comment Tuesday.

The Australian navy routinely turns back boats carrying asylum seekers from Africa, the Middle East and South Asia. However, these boats are usually returned to Indonesia, where most of the sea voyages begin.

Australia has come under criticism from human rights groups for its policy of refusing to allow refugees who attempt to reach its shores by boat to ever resettle in Australia.