The Australian Transport Safety Bureau said there were irregularities with onboard computer equipment on the A330-300 jet, which was forced to make an emergency landing at Learmonth air base near Exmouth, 1200km north of Perth.
The bureau said the jet`s pilots received electronic messages in the cockpit relating to irregularities with the plane`s elevator control system about 1.30pm local time.
The plane, carrying 313 passengers and crew from Singapore to Perth, then climbed 300ft before abruptly plunging nose-down.
Qantas said yesterday the cause of the sudden change in altitude was speculation and the emergency remained under investigation.
Fifty-four passengers and crew were injured in the mid-air emergency, including 13 who were airlifted with serious injuries to Perth on Tuesday night.
Andy Robertson, director of disaster management at Western Australia`s Health Department, said yesterday eight people were still being treated for conditions including spinal, neck and shoulder injuries at Perth`s two major hospitals. All were in a stable condition.
According to passengers, the jet had been cruising at 37,000ft before plummeting, sending passengers, crew and objects flying through the air.
"It just felt like going over a waterfall," passenger Mike Moir said. "It was pandemonium. It lasted 10 to 12 seconds. The plane then levelled out and there was another minor drop a few seconds later."
The second fall was reportedly not as dramatic as the first and fewer people were hurled around the cabin because they had clambered back into their seats to put their seatbelts on. One passenger, who did not want to be named, said that between the two sudden drops, the pilot told passengers there had been an "operational mechanical failure".
He said this sent "shudders of terror" through passengers, many of whom thought they were going to die.
Singaporean man Yip How Wong had just returned from the toilet and did not have his seatbelt on when the plane plunged, sending him flying into the roof of the cabin. "Everyone was screaming," Mr Wong said.
The impact of people and objects hitting the roof of the plane and overhead lockers caused significant damage, causing locker doors to crash to the floor and light coverings to be smashed.
Passenger Tim Ellett from Perth said he had his seatbelt on and was uninjured but the woman next to him was not restrained and went crashing into the ceiling of the cabin. She broke her leg and might have spinal cord injuries.
"She was in immense pain, she was in shock," Mr Ellett said. "She was saying `I think my back`s broken, I think my leg`s broken, can you put my leg back into place?`
"When we dropped again, there was a lot of screaming. One guy tried to get out of his seat, the girl behind me was in tears."
The bureau has sent seven investigators to Learmonth air base and has secured the jet`s flight data and cockpit voice recorders. The bureau will release a preliminary report on the emergency landing within about 30 days.
Australian and International Pilots Association president Ian Woods said yesterday that all planes experienced low-level turbulence during flights, but that air pockets could appear suddenly.
"There is probably an opportunity for the Government and regulators to run an awareness campaign (about wearing seatbelts)," Mr Woods said.
But he stopped short of saying it should be a legal requirement.
Turbulence, visible or not, is caused when winds moving in different directions, altitudes and speeds meet. At that boundary, the winds break down into unpredictable eddies of powerfully swirling air.
The turbulent boundary can be as much as 1km deep.
Additional reporting: Nicky Trup, Leigh Dayton
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