Up to 99 passengers aboard a Qantas Airways en route to Melbourne got the scare of their lives when the aircraft's suddenly depressurised mid-air and sent the plane rapidly plunging from its cruising altitude of 36,000 feet, down to 10,000 feet.

A Qantas spokesman told AFP that once the Boeing 737-400's pilot had detected that a malfunction on the plane's air conditioning system is causing problem, he was forced to initiate a rapid descent manoeuvre that saw the aircraft's cabin deploying its on-board oxygen masks.

Qantas said that the flight took off from Adelaide and was already travelling for some 30 minutes when the incident happened, which is not the first for the Australian national carrier during the past six months.

In November last year, a Qantas flight destined to Singapore suffered a blown engine over Indonesia, forcing the plane to turn back and return to Changi Airport. Shortly thereafter, more flights flown by the carrier were hit by mid-air incidents that soon compelled Qantas management to ground its entire fleet of Airbus A380s.

Qantas resumed flying its A380s in December following engine replacements and repairs performed by Rolls Royce, maker of the Trent 900 engines that power most of the Airbus planes in the employ of the company.

Yet another incident last week occurred last week as a Qantas flight bound for New York was re-directed to Fiji to fix a damaged valve that supplies fuel to the plane's engine. Nothing serious ensued on that flight, which is the same case with the Melbourne-bound Boeing aircraft.

Qantas said that the plane reached its destination without further issues and no passengers sustained any injuries from the mid-flight scare except for some panic issues, according to the airline management.

The company pointed to a glitch on the plane's air conditioning system as the likely culprit of the incident and an AFP report quoted a Qantas spokesman as saying that "there are two air conditioning systems on the aircraft and one of them failed at the cruising altitude, that's when they started to descend."

Soon enough, according to the spokesman, the other system conked out too due to overloading and the plane accelerated its descent to 10,000 feet, which the company stressed was a manoeuvre controlled by the plane's cockpit crew.