A dozen people treated for minor injuries after a Qantas Airbus hit severe turbulence during a flight from Hong Kong to Perth this morning have been discharged from hospital.

Horrified passengers said they saw people hurled around the cabin of the plane as it rose and fell about 250m due to "convective turbulence".

QF68, an A330-300 with 206 passengers and 13 crew on board, was four hours out of Hong Kong when it hit the turbulence over Borneo about 2.30am, a Qantas spokesman said.

"The aircraft most likely encountered what is known as convective turbulence, which led to it rapidly gaining around 800 feet in altitude before returning to its cruising altitude of 38,000 feet," Qantas spokesman David Epstein said.

One passenger described the incident as a brief but terrifying plunge that felt "like falling out of a 30-storey building".

Others told of their horror as they saw passengers flung around the cabin, some hitting the ceiling and crashing back into seats and the cabin floor.

After the injured received first-aid treatment on board, paramedics were waiting to treat them when the plane landed in Perth about 8am this morning.

Seven people were taken to Royal Perth Hospital, while a further five, including three children, were treated at Swan Districts Hospital, in Perth's east.

Spokeswomen for both hospitals said all 12 people were treated for injuries including bruising and neck and back pain and had been discharged by mid-afternoon.

Qantas said the aircraft's difficulties were not linked to any recent problems it has had with its A330 fleet, or the Air France A330-200 that crashed into the Atlantic Ocean on May 31, killing all 228 passengers and crew.

Mr Epstein said Qantas remained confident in the A330 aircraft and would work closely with the Australian Transport Safety Bureau to determine what might be learned from this incident.

He said there had been no problems with the aircraft and only minor damage was sustained, to two overhead cabin panels.

"There is nothing to link the aircraft to anything untoward," he said. Passengers said there were signs of panic when people who had not been strapped in were thrown around the cabin.

Chris Rose and wife Kerry, of Perth, said they knew what "the Air France passengers felt like".

"It's the worst turbulence I've every experienced," Mr Rose said.

"There were people flying around, bits and pieces flying around ... some (people) hit their heads.

"It was total freefall - absolutely. We're pretty happy to be back in one piece, that's for sure."

Another passenger, Ross, who did not want his last name published, said there was a lot of yelling and screaming when the turbulence struck.

He said there was a "sudden drop" and turbulence that lasted three to four minutes.

Another passenger, Perth mother Michelle Knight, who was travelling in a group of 12, including six children, said a friend had been taken to hospital after slamming into the ceiling of the cabin.

"My friend Vicky has cracked her head on the roof," Ms Knight said.

"She's got a big egg on her head and has gone to hospital complaining of neck pain."

She said Vicky's children were not strapped in when the plane dropped.

"They ended up in the aisle but they weren't injured," she said.

Ms Knight said there was some panic when there "was a big flash of light outside the window".

"I thought it was like lightning but they (the crew) said it was just the lights reflecting on the clouds.

"Things went everywhere. It was pretty scary and everyone was sort of panicking for a second, not knowing what to do or expect next."

Friends Therese Rosqvist, 26, and Nina Olsen, 25, on holiday in Perth from London said their first long-haul flight had been terrifying.

Ms Olsen said the pair was sitting next to a man who had been holding a baby, but not wearing a seatbelt, when the turbulence struck.

She said the man hit his head and began bleeding but the baby was not injured.

"I was shocked. I was like 'I'm going to die now'," Ms Olsen said.

"I'm still pretty shaken."

Ms Rosqvist said she was thankful she and her friend were wearing their seatbelts during the flight and both felt very "lucky to be alive".

Passenger Keith Huxtable said it felt as though the plane had "turned upside down" when it hit the air pocket.

"It appeared like we'd just dropped out of a 30-storey building," he said.

The incident was said to be similar to one during a flight from Singapore to Perth on October 7 last year, when almost 50 of the 313 people on board a Qantas A330-330 were injured as the plane plunged up to 2000m.

Mr Epstein said although some media reports had suggested the aircraft was travelling through thunderstorms at the time of the incident, there was no evidence of this.

Air safety authorities said two investigators had inspected the plane. Flight recorder data had been downloaded as part of the inquiry.

A preliminary report into the incident is expected within 30 days.

PERTH GEOFFREY THOMAS, JAYNE RICKARD AND AAP