A Qantas Airways plane made an emergency landing in Manila on Friday after plunging thousands of feet and losing cabin pressure during a flight from Hong Kong to Melbourne.

Passengers reported hearing a loud bang before the aircraft rapidly lost altitude and said the Boeing 747-400 had a hole the size of a mini-van in its undercarriage when it landed in the Philippine capital.

"It was an amazing experience, but not a good one," Glenyce Johnson, 47, told Reuters. "The aircraft plunged from around 40,000 feet to 25,000 feet."

Qantas confirmed the loss of cabin pressure and the rupture in the aeroplane's fuselage and said all 346 passengers and 19 crew disembarked safely. The aircraft was being inspected.

Johnson, who was travelling home to Australia, saw items flying out of the plane. Oxygen masks were released.

"I have to compliment the pilot for doing a good job, for safely landing the plane."

QF30 left Hong Kong at 9 a.m. (2 a.m. British time) and was due to land in Melbourne at 9:45 p.m. (12:45 p.m. British time). The flight originated in London.