Vietnamese detained at sea for month

A group of 46 Vietnamese asylum seekers were secretly detained a sea for a month before being returned to their country.

Immigration department secretary Michael Pezzullo rejected suggestions they were held aboard a prison ship in March, when he fronted a Senate committee overnight.

Commander of Operation Sovereign Borders, Major General Andrew Bottrell said there had been written assurance from Vietnam that provided a "level of comfort" about sending the asylum seekers back, although he acknowledged Australia does not keep track of returnees.