One of the flight’s pilots made a mayday call because the plane was low on fuel after a 15-hour flight.
The Boeing 787, carrying 194 people, landed safely, an Airservices Australia spokeswoman said.
The mayday call triggered a full emergency response shortly after Botany Bay Police Area Command were notified of the call, a New South Wales government statement said. Paramedics and fire crews were on standby on the ground.
While low fuel levels automatically trigger an emergency warning, the plane had enough fuel to safely land, Airservices Australia said.
Some major roads around the airport were shut down as a precaution.
United Airlines spokesman Madhu Unnikrishnan said the flight had a “mechanical issue” and “taxied to the gate and customers disembarked normally.”
After refueling, the plane took off again for San Francisco.
The Australian Transport Safety Bureau is investigating.
Civil Safety Authority spokesman Peter Gibson said the plane had about 40 minutes of fuel remaining.
“At this stage we don’t know why they used more fuel than planned,” Gibson said. “There can be many reasons, you can get unexpected head winds, stronger head winds, you need to divert around storm cells, you need to change altitude which can burn more fuel so there’s a number of reasons why you can use more fuel than planned.”