Diverted LAX-bound Qantas flight safely lands in Sydney after warning light in cockpit

A Melbourne Qantas flight bound for Los Angeles has safely landed in Sydney after being diverted when a warning light switched on in the cockpit.

Qantas says QF93 was about an hour into its flight from Tullamarine airport when an alert light turned on about 11am, indicating a loss of oil which required the engine to be set to idle.

"The captain made the decision to land in Sydney to have the fault fixed rather than continuing on the 14-hour flight," an airline spokesman said in a statement on Thursday.

The A380 landed safely about 1pm, after circling over New South Wales as it dumped fuel in preparation for landing. Its passengers currently remain on the plane.

A Qantas spokesman said the flight was circling the coastline but clarified the situation was not an emergency.

“There's been an indicator light on in the cockpit, and the captain has made the decision to divert the flight,” he said earlier.

The A380 aircraft holds 484 passengers, but there has been no confirmation of how many people are on board, however it is believed the plane was "busy".

Qantas said its engineers are now examining the aircraft and will repair the fault.

It is believed the engine wasn't shut down during the flight but continued to operate at idle thrust as a precaution.

The airline has apologised to passengers but says safety comes first.

"Our focus is now getting them on their way as soon as possible," the Qantas spokesman said.

Peter Gibson from the Civil Aviation and Safety Authority says the large aircraft would have been too heavy to land immediately.

"It would have been full of fuel for such a long flight so they (pilots) usually do zig zags to burn or dump the fuel," he told AAP.

"If it's an absolute emergency they'll just land but, if not, you don't want to risk causing damage to the aircraft."

Diversions aren't uncommon, Mr Gibson added.

"Over a normal week, I'd be surprised if there was not at least one flight which had to divert due to an indicator alert in the cockpit."