A Singapore Ministry of Foreign Affairs spokesman, responding to media queries on Wednesday, said the majority of the injured Singaporeans were given outpatient treatment, but 'a few' were under observation and said to be in stable condition.

He added that the ministry and the Singapore High Commission in Canberra have been in touch with the injured Singaporeans and their relatives, and given them all the necessary consular help.

"We will continue to monitor the situation and provide consular assistance to injured Singaporeans," said the spokesman.

Meanwhile an Australian investigator said on Wednesday that a computer glitch may have caused the Qantas jet to plunge mid-flight.

Police said 36 passengers and crew were injured, 20 seriously with broken bones, cuts and spinal injuries, when the Australian Airbus A330-300 suddenly began dropping nose-first Tuesday, forcing an emergency landing.

The plane was cruising at 11,200 metres when pilots received an automated warning of an 'irregularity with the aircraft's elevator control system,' an investigator told reporters in Canberra on Wednesday.

'The aircraft departed normal flight and climbed 300 feet,' said Julian Walsh, director of aviation safety with the Australian Transport Safety Bureau (ATSB).

'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,' he said.

Qantas, which has been plagued by incidents in recent months, said it was too early to speculate on the cause of the 'sudden change in altitude' of flight QF72 bound from Singapore to Perth carrying 313 passengers and crew.

But it confirmed that the plane climbed 300 feet before 'the nose dropped momentarily and the flight crew declared an emergency'.

News reports had speculated that clear air turbulence may have caused the plane to plunge, hurling passengers at the rear onto the roof, smashing ceiling panels and luggage lockers.

'It was horrendous, absolutely gruesome, terrible, the worst experience of my life,' said Jim Ford, of Perth, who said he thought he was going to die.

Several passengers said the plane fell 2,000 metres while cruising over the Indian Ocean just off the northwest coast of Australia.

'Passengers and crew not wearing seatbelts were flung around in the plane, some hit the ceiling,' said Nigel Court, who was travelling with his wife who was thrown onto the ceiling.

'Basically the plane just fell out of the sky,' another passenger told Sky News. 'It must have lasted 10 or 12 seconds, it just went straight down,' he said.

The jet made an emergency landing at an air force base near Exmouth in remote Western Australia when it became clear that there were serious injuries on board.

Numerous passengers and crew were taken off the jet by emergency services on stretchers and in wheelchairs.

Some 14 of the seriously injured were flown to Perth for treatment, while Qantas planes were sent to pick up the remaining passengers, many of whom were treated locally.

Investigators examining the grounded aircraft in Exmouth had removed its black box flight recorders to establish the cause of the incident, Qantas said.

An investigator from the French Bureau d'Enquetes et d'Analyses (BEA), which probes air incidents, and another from the European aircraft manufacturer

Airbus will also join the probe, Walsh said.

The investigation could take up to six months, but a preliminary report could be ready within 30 days, he said.