Linda Tse's Qantas flight was minutes from landing in Hong Kong when the plane began to violently shake before it dropped from 22,000 feet.

Passengers began to scream as phones, pillows and food flew around the cabin, hitting people in the head, the Bentleigh woman said.

"I thought I was dying. I thought it was going to crash and go down," she said.

"I screamed out as loud as I can."

The Qantas Boeing 747 flying from Melbourne had a "stick shaker" warning about 110 km southeast of Hong Kong last Friday.

The warning causes an aircraft's control stick to vibrate, alerting pilots it is about to stall.

Ms Tse, 60, said she believed the flight dropped a few thousand feet before it flew back up again.

Pilots disconnected the autopilot and manouevred the aircraft to safety, the Australian Transport Safety Bureau said. 

Fifteen people were injured in the terrifying two-minute ordeal.

Ms Tse, who was travelling alone, said she was left shaking, while other passengers were crying.

A young man sitting next to her offered comfort.

"It was mentally very scary," she said.  "I never experienced such an incident before in 40 years' travelling experience."

A flight attendant told passengers afterwards that it was caused by air turbulence, which was met with clapping.

However, Ms Tse, a Qantas frequent flyer, said she suspected there was a fault with the plane.

"They said nothing happened," she said. "I don't believe what Qantas' flight crew says.

"I really lost confidence with Qantas at the moment." 

The safety scare is so rare and serious that most pilots go their whole career without encountering such a scenario, an aviation expert said.

The ATSB, which is investigating, said it is treating it as a "serious incident", meaning there were indications that an accident causing loss of life or aircraft damage nearly occurred.

It said a report would be released within months after it interviewed the flight crew.

Qantas said the injuries were "minor", with two passengers taken to hospital as a precaution and later released.

"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.

With AAP