Air safety experts were investigating whether an explosion inside some luggage or a broken panel punched a hole in a Qantas airliner, forced to make an emergency landing in Manila.

Experts were focusing on the two theories as they investigated the dramatic rupture on the Boeing 747's fuselage as it flew from Hong Kong to Melbourne with more than 300 passengers and crew on board, said a source close to the investigation.

"They were very lucky.

"While it is too early to say what actually caused the hole, we will be looking at two possibilities ... something exploded in one of the bags or a panel came loose on the fuselage," the source told AFP.

The source said the explosion might have been caused by a pressurised container inside a piece of luggage, saying a bomb was unlikely.

The aircraft was at 29,000 feet (9,000 metres) when the crew was forced to make an emergency descent, dropping nearly 6,000 metres, after a section of the fuselage separated and resulted in rapid decompression of the cabin.

Captain John Bartels made a safe emergency landing at Manila airport, and no one was injured in the incident, which at least one passenger has described as an "absolutely terrifying" ordeal.

An urgent investigation is underway into what ripped a three-metre hole in the fuselage near the right wing.

Investigators from Qantas and the Australian Transport Safety Bureau have arrived in the Philippines to inspect the plane, officials said.

The local Air Transport Office is also looking into the incident, Manila international airport general manager Alfonso Cusi said.

'No explosion'

Cusi said he did not believe an explosion caused the incident.

"Our air safety people are looking into it and from the looks of it, it doesn't look like an explosion.

"We still think it is a technical problem on the plane," Cusi added.

The Qantas office in Manila declined to comment on the investigation, referring all inquiries to its office in Sydney.

The Philippine Daily Inquirer said pilot Bartels had told Philippine aviation authorities that he started dumping fuel on his way to Manila for the emergency landing.

The deputy manager for operations at Manila airport, Octavio Lina, was also quoted by the paper saying that "the flooring gave way, exposing some of the cargo beneath".

"The ceiling around the area also collapsed," he said.

But despite the seriousness of the investigation, there was no need to ground all Boeing 747 aircraft around the world, ATSB Director of Aviation Safety Investigation, Julian Walsh said.

"These aircraft have a very good safety record," he told reporters.

"There's no information to suggest that such a recommendation should be made.

"That would be something the Civil Aviation Safety Authority, as the airworthiness authority would have to make a decision on.

"However, there's no information that we have to make any sort of recommendation about the airworthiness of this aircraft."

Qantas chief executive Geoff Dixon said the plane's maintenance log book showed it underwent a "D Check" - which is the major check for aircraft - in Sydney in 2004 followed by a lower-grade C Check in 2006, followed by another C Check this year, both at Melbourne's Avalon airport.

"They were routine checks and nothing untoward was found," he said.

"The D checks and the C checks are designed to look for corrosion ... our preliminary checks on this indicate that there was no corrosion anywhere near where this hole occurred in the aircraft."

Mr Dixon said corrosion could occur on an aircraft as early as a year after it was introduced into service.

"I would have thought there would be corrosion in just about every aircraft in every airline in the world, at some stage," he said.

"That's what the checks are for."

Mr Dixon also praised the pilots, cabin crew and passengers who, he said, performed a text-book emergency landing.

"For the passengers involved it must have been a very frightening situation," he said.

Mr Dixon also moved to counter union and media claims that increasing the use of maintenance crews in Asia had contributed to the incident.

"I don't think we do have falling maintenance standards," he said.

"We have one of the highest standards of safety and operations of anywhere in the world."