A Qantas flight departing Tokyo bound for Sydney was forced to divert and land in Cairns after passengers heard a "loud bang" from their aircraft's engine.

Qantas Flight QF26 from Tokyo to Sydney operated by a Boeing 747-400 made an unscheduled landing in Cairns after pilots received an indication there was an issue with one of the engines.

According to the Cairns Post, passengers on board the flight reported hearing a "loud bang" while mid-flight in the early hours of Sunday morning.

One passenger, Desmon Du Plessis, claims he saw "orange flames" and "sparks" from his seat above the wing.

"I was sitting at the window and there was an incredibly loud bang, and when I looked out there was an orange flame and then sparks, it was like white," he told the Cairns Post.

"And then everything started shaking, and you could hear the engine die straight away."

Mr Du Plessis said the captain addressed the passengers about the incident and advised of the diversion to Cairns.

"It was a bit scary," Mr Du Plessis said.

"We were deeply, deeply relieved (to land safely)."

Another passenger, Andrew Mak, said he heard a "bit of an explosion" on the flight.

"We didn't quite know what was happening. There were quite a few vibrations occurring from that initial explosion," he said.

In a statement to news.com.au, Qantas said the engine did not experience an "explosion" and was shut off by the pilots in line with standard procedures. Debbie Slade, Qantas fleet safety captain, said despite passenger concern there was no safety risk during the flight.

"Our Tokyo to Sydney flight diverted to Cairns due to an issue with one of the engines," Ms Slade said in a statement.

"In line with standard procedures, the pilots shut down the engine and the flight diverted to Cairns. While customers may have heard a loud bang, there was never a safety risk with the flight. These aircraft are designed to safely operate on three of the four engines."

Ms Slade said the aircraft had a "normal landing" and the aircraft was now undergoing an inspection with engineers.

The incident on-board the aircraft occurred five hours after departing from Tokyo, and the plane landing in Cairns just after 5am local time.