A Qantas jet was forced to make an emergency landing at Adelaide airport overnight because of a mechanical problem during a flight to Melbourne, reports said.

The Herald Sun newspaper quoted unnamed passengers as saying a door opened causing "chaos" in the cabin of the Boeing 737-800, which left Adelaide at 6:08pm and returned safely 37 minutes later.

But airline sources said only the door covering the wheel bay was not closed properly after take-off, the Herald Sun said.

No danger after plane aborts flight: Qantas

A Qantas spokeswoman said passengers aboard the Qantas jet were never in any danger.

"Qantas flight 692 operating between Adelaide and Melbourne performed a routine 'air turn-back' shortly after take-off, due to an indication of one of the landing gear doors failing to retract," a spokeswoman told AFP.

"The aircraft landed without incident and all passengers were accommodated on other flights. There was no safety risk at any time," she said.

The passengers were informed of what was happening as the plane returned to Adelaide, before they were transferred to other flights, she said. 

Troubled time for Qantas 

The incident came three days after a Qantas Boeing 747-400 en route to Melbourne from Hong Kong was forced to make an emergency landing in Manila after a large hole was ripped in its fuselage.

The plane, which had originated in London and was carrying 365 passengers and crew, plunged 6,000 metres before stabilising, then made an emergency landing in Manila on Friday.

Australian Air Transport Safety Board (ATSB) investigators are focusing on whether an oxygen bottle used for emergency back-up for the cockpit exploded mid-flight, tearing a three-metre (10-foot) hole in the Boeing's fuselage.

One of two such cylinders is missing from the plane, investigators said.

Airline and air safety officials said it would be the first time an onboard oxygen bottle had exploded on a passenger jet in mid-air.

Qantas is carrying out urgent inspections of oxygen bottles on its entire Boeing 747 fleet after the terrifying mid-air drama, despite hearing from the plane's maker that no such incident had occurred previously.

"Boeing advises that no, they have not had one of their aircraft with an oxygen tank disintegrating," Qantas chief engineer David Cox told reporters.

But Cox refused to speculate on whether an exploding oxygen bottle was to blame for rupturing the Boeing's fuselage in an incident that Qantas executives acknowledged ended in a lucky escape for passengers.

"We don't know that was the root cause so that's why we're not going to speculate. The fact that that has never happened may be relevant, it may not be relevant," Cox said.

Air safety officials confirmed that if an exploding oxygen bottle is proved to have blown a hole in the jet, it would mark the first time such an incident has been recorded in a large passenger plane.

"As far as we can determine this has never happened before on a passenger aircraft," Civil Aviation Safety Authority spokesman Peter Gibson said.

"There's no reports of it anywhere, so it's very, very unusual and obviously understanding why that happened will be absolutely critical to making sure it can't occur again," he told the Australian Broadcasting Corporation.

Metal fatigue in the cylinder, a failure of the regulator valve, something hitting it and puncturing the bottle, or overheating were among possible causes the ATSB would look at, he said.