Australia's air safety watchdog is investigating a serious mid-air incident involving a Qantas jet, after 15 people were injured when it suffered a so-called "stick shaker" warning.

The drama unfolded about 110km southeast of Hong Kong on April 7, after flight QF29 took off from Melbourne.

Pilots reported feeling "airframe buffeting", a potential warning the plane is about to stall, while "holding" at 22,000 feet.

The flight crew also received a "stick shaker activation", the Australian Transport Safety Bureau said. The stick shaker device causes the aircraft's control stick to vibrate noisily to warn the pilot of an imminent stall.

The ATSB classified the Boeing 747's incident as serious and said a report would be released "within several months" after it interviewed the flight crew.

"The flight crew disconnected the autopilot and manoeuvred the aircraft in response," it said.

"Fifteen passengers received minor injuries."

The incident is understood to have lasted about two minutes and while the flight landed normally in Hong Kong, an ambulance met passengers as a precaution and one person was taken to hospital for assessment.

Qantas said customers experienced "unexpected in-flight turbulence" during the trip.

"We notified the ATSB of the occurrence, and our own teams are also reviewing the event," a spokeswoman said.

The plane was inspected by engineers before it flew out of Hong Kong again that night.

AAP