The Airbus A330-300, which had taken off from Washington Dulles International Airport in Virginia, was heading for Munich.
The plane LH-415 was 70 miles south-west of Sydney when the crew alerted ground control after a passenger reported there was a burning smell, aviation information website AeroInside said.
The crew diverted to Boston and landed 70 minutes after the diversion at 2:06 AM local time on Setember 8, but details have just come to light.
"The occurrence aircraft remained on the ground in Boston for 16 hours, then continued the flight and reached Munich with a delay of 18 hours," it said.
According to a Lufthansa spokesperson, once the flight was on the ground, technicians inspected the aircraft and confirmed the source of the smell to be an overheated coffee machine.