New Qantas scare with Fiji flight aborted

Qantas has been forced to turn back a flight from Sydney to Fiji because of potential mechanical troubles, a day after a service from Auckland issued a mayday following an engine shutdown.

Qantas on Thursday said QF101, a Boeing 737 bound for Nadi, returned to Sydney after an onboard "fault indicator" about a possible mechanical issue.

"The pilots followed standard procedures and the aircraft has landed normally in Sydney. Engineers will examine the aircraft," a Qantas spokesperson told AAP.

The plane returned without emergency or priority landing and the fault indicator did not relate to an engine issue, Qantas said.

Online flight tracker Flightradar24 showed the plane making loops off the east coast before heading back to Sydney.

Earlier, transport safety investigators confirmed they would analyse QF144's cockpit voice recorder and flight data after an engine failure on the flight from Auckland on Wednesday.

The pilot of QF144 - a Boeing 737 - shut down the engine and made a mayday call over the Pacific Ocean, before landing safe at Sydney's Kingsford Smith Airport about 3.30pm.

Qantas said all 145 passengers disembarked normally and shutdowns were rare, with pilots trained to manage them safely.

Passengers reported turbulence on the flight but said they were unaware of a mayday alert until they landed.

The Australian Transport Safety Bureau said three officers had started the "evidence collection phase" of a probe, launched hours after the emergency.

"At the ATSB's request the operator has quarantined the aircraft's cockpit voice and flight data recorders. Once downloaded, information from those recorders will be analysed at the ATSB's technical facilities in Canberra," the ATSB said in a statement.

It said the officers would likely also interview flight crew, review operator procedures and analyse weather data.

The probe would also examine any relevant engine components and could involve a "tear-down inspection of the engine", the ATSB said.

The investigation would work to establish a sequence of events with the aim to determine any underlying safety issues, the agency added.

"If at any time during the course of this investigation we uncover any critical safety issues, we will immediately share those ... so timely safety action can be taken," it said.

Qantas said its investigations into QF144 continued.

On Wednesday, Australian and International Pilots Association vice president Mark Hofmeyer commended the pilots for making safe decisions under pressure.

"At the end of the day, it was a safe outcome," Mr Hofmeyer told AAP.

Federal Transport Minister Catherine King praised Qantas' safety record after the scare, that had 100,000 people tracking the flight online.

"Well done to the highly experienced crew for getting the plane safely home," she tweeted.

Qantas has never had a passenger lost on a jet aircraft in its history.