In a preliminary report made public on Thursday, the Australian Transport Safety Bureau (ATSB) said a "sequence of events" led to the engine failure.
It started with an oil fire caused by a manufacturing defect in an oil feed pipe.
"That defect resulted in fatigue cracking in the pipe, so that oil sprayed into an engine cavity where it ignited because of the high air temperature," the report said.
The oil fire then weakened a turbine disc in the aircraft's second engine.
"As a result, the disc separated from its shaft, increased its rotation speed and broke into several parts," the investigation found.
Sections of the fractured disc and other engine components went on to penetrate the aircraft's left wing, along with other areas of the plane, causing major structural damage.
The final ATSB report on the incident is expected to be released in May.
The plane's engine manufacturer Rolls-Royce said it was working closely with the ATSB to make sure that all issues were effectively addressed.
"Each time an incident happens the aviation industry learns lessons," said Rolls-Royce spokesman Richard Hedges.
"These are embedded in the rigorous safety procedures and standards of regulation which make flying an extraordinarily safe form of transport."
The report said Rolls-Royce had already revised manufacturing procedures and risk assessment, and the investigation would monitor the progress of those initiatives.
The aircraft is currently in Singapore awaiting repair.
Qantas grounded its entire superjumbo fleet following the incident while it carried out its own investigation.
- AP
