The airline, which is locked in heated talks with pilots, engineers and baggage handlers on wages and conditions, said the plane carrying 115 passengers landed safely.

"There was no emergency. The landing gear couldn't retract," A Qantas spokeswoman said.

"The aircraft landed safely. There were no safety issues at any time and no issues with the cargo door."

The Australian Broadcasting Corporation quoted a Northern Territory politician on the flight as saying the cargo door opened just after take-off.

"When we heard the first noise of something that clearly was different from a normal plane flight, I turned to one of my colleagues next to me and said, well, clearly something is wrong here," said Adam Giles.

"I think everybody knew that," he said.

The incident came two days after a Qantas pilot was forced to shut down one of his plane's engines in mid-flight after it developed an oil pressure problem.

The Dash 8 aircraft carrying 23 people from Sydney to Lord Howe Island landed safely.

A Sydney-bound Qantas flight also had to turn back to Bangkok on Monday after a bang was heard on board.