The Civil Aviation Administration of Vietnam (CAAV) says safety rules at Jetstar Pacific Airlines, which is part-owned by Qantas, have been breached.
Among the dangerous practices identified by the authority was the removal of a defective anti-icing pipe that, instead of being replaced, was allegedly welded and placed back in the aircraft by an unauthorised contractor.
It has been reported that the CAAV report also found Jetstar had illegally dismissed an Australian engineer who tried to notify senior manager in Australia of the safety concerns.
The safety queries come as two Australian executives of the airline have been banned from leaving Vietnam while the Vietnamese Government undertakes an investigation into the airline's losses of $33.5 million over fuel price hedging.
Jetstar Pacific's former chief executive, Luong Hoi Nam, has been put in prison over matters relating to his tenure at the company last year.
The Transport Workers Union (TWU) says it has been investigating the Qantas group of companies for alleged safety breaches for several months and has requested a meeting with delegates and employees from the company in Vietnam.
"The unions want to investigate how a Qantas subsidiary is behaving overseas and whether or not they are flouting international aviation safety laws,'' TWU spokesman Tony Sheldon said today.
He said union members also wanted to know whether Vietnamese, Australian or International Labor Organisation standards were being followed.
"Qantas is an Australian icon - with a majority owned by the Australian community,'' Mr Sheldon said.
"We do not want to see its brand cheapened because executives are putting their bonuses before safety and security.''
If the Vietnamese government agreed to the union's request, an independent observer would also go to Vietnam to investigate the CAAV's claims, Mr Sheldon added.
Qantas and Jetstar have not responded to requests for comment.