A Melbourne to Los Angeles Qantas flight is back in the air and en route to the US after the pilot was earlier forced to divert to Sydney when an indicator suggested the engine was losing oil.
The QF93 flight left Tullamarine airport before 10am on Thursday but about an hour later an alert light turned on indicating a loss of oil.
The engine was set to idle - but not shut down - and the captain decided to land in Sydney after circling east of the city to reduce the plane's fuel load.
The Airbus A380 landed at 1.05pm and was inspected by engineers. Passengers waited nearly three hours before it departed again to continue the 14-hour journey to Los Angeles.
"Passengers were very understanding ... food and water were provided to all," a Qantas spokesman told AAP.
The flight departed Sydney Airport at 3.45pm and is due to arrive in Los Angeles about four hours later than originally scheduled.
The airline has apologised to passengers but says safety comes first.
"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.
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."
