Powerful signals from a US-Australian naval base might have caused a Qantas jumbo jet to to plummet hundreds of feet last month, injuring scores of passengers, say investigators.

The Australian Transport Safety Bureau (ATSB) is also examining whether a passenger's laptop might have interfered with the plane's computer system, according to the preliminary report into the incident where the passenger flight from Singapore to Perth suddenly nosedived from the sky twice in the space of minutes.

ATSB capability director Kerryn Macaulay said on Friday that investigators were looking into whether outside interference caused the plane’s computer system to malfunction mid-flight.

“Possible external sources of electromagnetic interference are being explored and assessed, including from the Harold E. Holt very low frequency transmitter near Exmouth, WA, and from portable electronic devices on board the aircraft,” she said.

However Ms Macaulay added that transmissions from the naval base were “unlikely” to be the source of the problem, that it was more likely to have been a glitch in the plane’s computer system units, known as Air Data Initial Reference Units (ADIRU).

The manufacturer of the three ADIRUs on the Airbus will begin testing the components in the US on Monday.

"A carefully prepared test plan is currently being finalised in anticipation of this complex work to ensure the investigation team has the best possible chance to understand what led to the pitch-down events," Ms Macaulay said.

The pilots issued a mayday call when the Airbus, with 303 passengers onboard, plunged 1000 ft in just over a minute on October 7.

They managed to land the plane safely at Learmonth airport near the remote coastal town of Exmouth, 1,200km north of Perth. However 13 passengers and one crew member were seriously injured and some were airlifted to hospital in Perth.

The top-secret naval base at Exmouth transmits signals to US and Australian navy ships, including nuclear submarines, in the Pacific and Indian oceans.

The station, on Australia's north-west coast, is the most powerful transmission station in the Southern Hemisphere and has been a frequent target of peace protests since it opened in 1963.

The tallest of the station's 13 radio towers stands 1,270 feet tall — higher than the top floor of New York's Empire State building.

The Defence Department said it agreed with the preliminary report that the transmitter was unlikely to be the cause.

The transmitter “emits very low frequency transmissions and from a significant distance from the aircraft flying overhead,” the department said in a statement.

“This indicates that it is unlikely to have caused sufficient currents in the system to have caused problems with the A330 systems,” it added.

Qantas said its own investigations found the likely cause of the emergency was a “manufacturer fault” in the computer units.

Last month the ATSB and Airbus issued emergency guidelines to airlines worldwide operating the Airbus A330-300 in the event of a similar emergency.

The ATSB is investigating the incident alongside its French and US safety investigation counterparts.

The incident was one of a series which have recently plagued Qantas, Australia’s flagship international airline, long considered one of the safest in the world.

The most serious occurred in July when a Qantas jumbo jet, en route from London to Melbourne was diverted to Manilla after a gas cylinder on board exploded mid-air, ripping a massive hole in the fuselage. Miraculously nobody was seriously injured.

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