Julian Walsh, acting executive director of the Australian Transport Safety Bureau, released an interim report Friday into the investigation of the emergency that occurred over the South China Sea on July 5.
Walsh said one of the seven emergency oxygen cylinders below the cabin floor had exploded. He did not say why.
"On the basis of the physical damage to the aircraft's forward cargo hold and cabin, it is evident that the number 4 passenger oxygen cylinder sustained a failure that allowed a sudden and complete release of the pressurized contents," Walsh told reporters in releasing the report.
The Boeing 747-438 aircraft, carrying 365 people, immediately lost cabin pressure after the explosion blew a hole 60 inches long in the fuselage. The plane - en route from London to Melbourne, Australia - rapidly descended thousands of feet and flew to Manila, 295 miles away.
No one was injured in the incident, but questions were raised about the much-lauded safety of Qantas Airways, which has never lost a jet aircraft because of an accident.
In the weeks after the incident, Qantas planes experienced a number of other problems, including a loss of hydraulic fuel that led to an emergency landing, failure of landing gear, and detached panels.
The problems prompted the Civil Aviation Safety Authority, Australia's aviation agency, to launch a review of Qantas Airways' safety standards.
Qantas earlier this month temporarily pulled six planes from service because of irregularities in maintenance records. Qantas said it was a record-keeping issue and there were no safety implications for the aircraft.