TWO Qantas executives have been prevented from leaving Vietnam as part of an investigation into losses incurred by the Vietnamese arm of the airline's no-frills brand, Jetstar Pacific.

Vietnamese authorities have been questioning Tristan Freeman, Jetstar Pacific's chief financial officer, and Daniela Marsilli, its chief operating officer, in Ho Chi Minh City.

The investigation escalated on Thursday when economic police arrested Jetstar Pacific's former chief executive, Luong Hoai Nam.

The Australians have been banned from leaving the country since Christmas while they are interrogated.

The Qantas chief executive, Alan Joyce, said the pair were helping the Vietnamese Government understand decisions that led to losses of $31 million in 2008 in fuel purchases.

Airlines hedge their bets on future fuel prices to try to control costs. Many lost money when prices sank during the global financial crisis.

''Vietnam is a country that is certainly opening itself up to economic development. Some of these transactions and processes are new to the country,'' Mr Joyce told ABC radio. ''We're obviously worried about our people on the ground … and we are working with the Australian Government.''

Australian diplomats in Canberra and Hanoi have pressed Vietnamese officials to explain its refusal to allow the two Australians to leave the country.

The Department of Foreign Affairs and Trade said it raised the issue with the Vietnamese ambassador, Nguyen Thanh Tan, in Canberra on December 24. Australian embassy officials in Hanoi have since pressed Vietnamese officials for an explanation.

Mr Nam, whom the Communist Party appointed to the top job five years ago, suddenly resigned on November 18. He was later banned from leaving Vietnam.

Vietnamese media alleged yesterday that Mr Nam, 47, was suspected of ''being irresponsible'' and ''causing serious losses''.

The families of the two Australian executives returned to Australia just before Christmas.

Mr Freeman, a long-standing executive with Qantas and Jetstar, is married with two young children. Ms Marsilli, who ran Adelaide charter airline National Jet before joining Qantas, is married with at least one child.

Ms Marsilli's father Sergio said from Adelaide last night that his daughter had phoned him from Vietnam and is well. Neither Foreign Affairs nor Qantas have contacted him about her situation, he said. ''Not a word.''

No charges have been laid against Ms Marsilli or Mr Freeman.

Qantas has built up its stake in Jetstar Pacific to 27 per cent since taking an initial 18 per cent investment in July 2007.

The remainder is owned by the Government-owned State Capital Investment Corporation, the Saigon Tourist Company and Mr Nam.

with AAP