The Australian Transport Safety Bureau (ATSB) on Wednesday said an "irregularity" in one of the plane's computers caused the dramatic altitude change on Tuesday that hurled passengers around the cabin.
The pilots of the Singapore-Perth flight made an emergency landing in Learmonth, near Exmouth in WA's north. Passengers seriously injured in the incident were airlifted to Perth.
ATSB director of aviation safety investigation Julian Walsh said the plane was travelling at 37,000 feet and 177km north of Carnarvon when the incident occurred.
Walsh said the pilots received messages about "some irregularity with the aircraft's elevator control system", before the plane climbed 300 feet and then nose-dived.
"The aircraft is then reported to have abruptly pitched nose down," Walsh said.
The elevator control system is the control surfaces on the tail of the jet which allow pilots to guide the plane's altitude. It is vital for stable flight.
Qantas said 74 people were injured in the incident, while the West Australian Health Department said 51 had been treated by three hospitals in Perth for fractures, lacerations and suspected spinal injuries.
The airline refused to speculate on the cause of the nose-dive as it began its own internal investigation.
The Airbus A330's cockpit voice recorder and flight data black box have both been removed, Qantas chief executive officer Geoff Dixon said.
"Our primary concern remains the welfare of our passengers and crew on board the flight, and we are focused on doing everything possible to assist them," Dixon said in a statement.
Airbus was sending its own investigator from its French headquarters in line with standard practice, a spokesman said.
The safety bureau's comments about an irregularity seem to rule out initial talk of turbulence being responsible.
However, Qantas pilot and Australian and International Pilots Association president Ian Woods said it was a real possibility.
"When you cross those jet streams as you do from Singapore to Perth ... you run across the transition boundary," Captain Woods told ABC Radio on Wednesday.
"It's at that point where you're crossing from smooth air to fast-flowing air, that there can be quite unexpected and significant turbulence."
The incident hasn't helped Qantas as it fights perceptions of a worsening safety record following a series of emergency landings, delays and flight turn backs.
The Civil Aviation Safety Authority (CASA) has been taking an increased interest in the flying kangaroo of late, ordering a full audit of the airline's three major planes - the Boeing 747-400, 737-400 and 767-300.
Despite perceptions, the authority says there has been no great increase in the number of safety incidents at the airline, regularly named the safest in the world.
"Qantas is a safe airline, and CASA has no doubt about that," deputy chief executive officer operations Mick Quinn said last month.
The latest incident even garnered media attention in the US.
"There are parts of the world in which safety is at such a high level that even the smallest scratch seems to be a warning sign of something dire," a former member of the US National Transportation Safety Board, Richard Healing, told the New York Times.
