Australian passengers have recounted terrifying stories of debris flying through the cabin of a Qantas plane after a "gigantic" hole opened up in its belly at 29,000 feet.

Passengers said they realised how lucky they'd been when they saw the size of the hole near the wing after pilots made an emergency landing in Manila today.

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The hole was so large that it extended from the cargo hold into the passenger cabin, sending debris and freezing wind swirling, Melbourne woman Marina Scaffidi said from Manila.

Scaffidi described hearing a "big bang" as the plane flew over the South China Sea en route from Hong Kong to Melbourne, sending "wind swirling around the plane".

Another Melbourne woman June Kane described how parts of the plane's interior broke apart in the depressurised cabin.

"There was a terrific boom and bits of wood and debris just flew forward into first (class) and the oxygen masks dropped down," she told ABC Radio from Manila airport.

"I'm looking at the plane now and ... just forward of the wing, there's a gaping hole from the wing to the underbody," she said, adding that baggage was hanging out.

"It was absolutely terrifying, but I have to say everyone was very calm."

The Australian Transport Safety Bureau today said it was sending our investigators to Manila to inspect the Boeing 747-400, which Qantas said had been carrying 346 passengers and 19 crew. No one was hurt.

"At approximately 29,000 feet, the crew were forced to conduct an emergency descent after a section of the fuselage separated and resulted in a rapid decompression of the cabin," the bureau said.

The crew brought the plane down to 10,000 feet and requested an emergency landing in Manila, where emergency crews were on hand to watch it come in.

Qantas chief executive Geoff Dixon said the airline was also sending its engineers to Manila.

Manila airport operations officer Ding Lima told local radio that many passengers were extremely distressed as they got off the plane.

"Upon disembarkation, there were some passengers who vomited. You can see in their faces that they were really scared," he said.

He said part of the plane's flooring and ceiling had given way during the emergency and it was lucky that no one had been hurt.

Brendan McClements, the chief executive of the Victorian Major Events Company, described how wind was sucked from the cabin after passengers heard a loud bang.

"There was a sort of rapid expulsion of wind. It went out of the plane, the air got sucked out, the oxygen masks dropped down and we put them on," he told AAP.

He said the the cabin was "noisy and very windy" after the bang but the plane did not feel out of control at any time.
When the plane landed "there was a very large hole that wasn't there when we took off in Hong Kong," McClements said.

"I was very happy I saw that when I was on the ground and not when I was in the air."

He had high praise for the cabin crew who kept passengers calm.

"Everyone gave them a round of applause as we landed."

British man Phil Rescall said he and other passengers realised how lucky they were when they saw the size of the hole in the plane's underbelly just in front of the right wing.

"You see the hole and you realise we were very lucky," he told AFP.

"Some people were crying, some people were pretty shaken when they saw the hole."

Another English passenger, Robin McGeechan, 42, said that despite the drama, there was little panic.