Sydney - An auto-pilot computer malfunction, a recalcitrant passenger's laptop or air turbulence were Thursday offered as reasons for a Qantas passenger jet en route from Singapore and Perth plunging in altitude and injured 46 passengers.

Seven passengers from Tuesday's Flight QF72 remain in hospital in Perth, where they are being treated for broken bones, spinal injuries and lacerations.

The Airbus A330 dipped around 8,000 feet (2,440 metres) in about 10 seconds, an hour from its destination. It made an emergency landing at Learmonth military base, 1,260 kilometres north of Perth.

Phil Gleeson, from legal firm Slater and Gordon, was drumming up business among the 303 passengers for a compensation claim against Qantas.

"Even a situation where there's been violent turbulence, I think there's scope to argue that that's an accident," Gleeson said.

Qantas has given passengers free return air tickets to London and offered to pay their medical expenses.

"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."

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."

There were suggestions that a passenger's laptop or mobile had interfered with the instrumentation of the Airbus - or that clear air turbulence was responsible for an incident passengers said came without any warning. (dpa)