Sydney - A computer malfunction could be the reason why a Qantas passenger jet en route from Singapore and Perth plunged in altitude and injured 46 passengers, investigators said Wednesday.

Passengers aboard Flight QF72 said the Airbus A330 dipped 8,000 feet (2,440 metres) in about 10 seconds on Tuesday before making an emergency landing at Learmonth military base, 1,260 kilometres north of Perth.

Julian Walsh, director of safety at the Australian Transport Safety Bureau told national broadcaster ABC that the flight crew received a message of an irregularity in the elevator control system prior to the sudden loss of altitude.

"The aircraft departed normal flight and climbed 300 feet," Walsh said. "The aircraft did that of its own accord and then whilst the crew were doing the normal actions in response to that not-normal situation the aircraft then pitched down suddenly and quite rapidly."

Passenger Mark Bell said the frightening plunge came without any warning.

"The poor kid next to us, we watched him hit the ceiling and sit there for about three seconds until his dad dragged him back into his seat," he told the ABC on his arrival at Perth airport.

None of the 36 passengers in hospital has life-threatening injuries, but they did suffer cuts, bruises and broken limbs.

Jason Armstrong, of Perth's Sir Charles Gairdner Hospital, said some passengers in his care may have suffered spinal injuries but it was too early to say how serious they were.

"There were minor cuts and scratches to the head and minor problems related to muscular injuries in the neck - as you can imagine if you were suddenly woken up and you were attached to the ceiling," he said.