SYDNEY, AUSTRALIA - FRAGMENTS of an oxygen cylinder blasted into a Qantas jet's passenger cabin mid-flight with enough force to shear off a door lever when an explosion punctured the jet's body, investigators said on Wednesday.

The Boeing 747-400, with a huge hole in its fuselage, then made an emergency landing in Manila last Friday without landing instruments and anti-skid safety systems, the Australian Transport Safety Bureau said.

Bureau spokesman Julian Walsh said the cylinder fragments shifted the aircraft door's handle halfway to the open position after exploding through the floor of the passenger cabin, but the door held fast.

'It appears that part of an oxygen cylinder and valve entered the passenger cabin and impacted the number two right door frame handle, thereby moving the handle halfway towards the open position,' Mr Walsh told reporters.

'However, the door handle mechanism has been sheared, as it is designed to do when an attempt is made to open the door in flight.'

The initial findings appear to back the theory that an exploding oxygen cylinder was responsible for punching a three-metre hole in the fuselage of the plane, which had 365 people on board.

Australian investigators in Manila this week said one of the six oxygen cylinders in the forward cargo hold was missing and that metal fragments had been recovered from the jet.

The aircraft remains grounded at Manila airport after the scare shortly after departing from Hong Kong for Melbourne.

Mr Walsh said the plane's three instrument landing systems and an anti-skid device were not working when it made the emergency landing, although the engines and hydraulics were working normally.

'The approach to Manila airport was conducted in visual conditions, and it should be noted that the pilot had other navigation instruments available,' he said.

Mr Walsh said the probe was also looking at whether there was enough oxygen for passengers as the plane made a 6,000 metres emergency descent that took five-and-a-half minutes.

'The investigation team is still examining the oxygen system, including liaising with the manufacturer to determine if the oxygen flow was adequate for the five-and-a-half minute descent to 10,000 feet,' he said.

Sydney's Daily Telegraph reported Wednesday that passengers complained children were turning blue during the descent.

Walsh said 'a small number' of oxygen masks failed to deploy from the aircraft's ceiling during the emergency and that the bureau would question the 346 passengers on the flight about their experiences. -- AFP