Dozens injured on Sydney-bound flight

An Air Canada flight headed to Sydney, has been diverted to Hawaii after sudden turbulence injured 37 people, nine of them seriously.

Passengers on a Sydney-bound Air Canada flight had to rebook flights after a diversion to Honolulu. (AAP)

Intense turbulence struck an Air Canada flight to Australia on Thursday and sent unbuckled passengers flying into the ceiling, forcing the plane to land in Hawaii.

The flight from Vancouver to Sydney encountered "un-forecasted and sudden turbulence," about two hours past Hawaii when the plane diverted to Honolulu, Air Canada spokeswoman Angela Mah said in a statement.

"The plane just dropped," passenger Stephanie Beam told The Associated Press. "When we hit turbulence, I woke up and looked over to make sure my kids were buckled. The next thing I knew there's just literally bodies on the ceiling of the plane."

A woman behind her hit the ceiling so hard that she broke the casing of an oxygen mask, said Beam, of Colorado Springs, Colorado.

Of the 37 passengers and flight crew members injured, nine had serious injuries, emergency responders said. Thirty people were taken to hospitals.

Honolulu Emergency Medical Services Chief Dean Nakano said the injured ranged in age from children to the elderly. Customs agents and emergency responders met passengers at the gate at the Honolulu airport to ensure they could get medical attention quickly.

Honolulu Emergency Services Department spokeswoman Shayne Enright said injuries included cuts, bumps, bruises, neck pain and back pain. More than two dozen people were taken to hospitals, she said.

"I watched a whole bunch of people hit the ceiling of the plane," said another passenger Alex MacDonald. "A couple of the air hostesses were bringing food out at the time, and they hit the roof as well."

Passenger Luke Wheeldon told Honolulu news station KTIV about half the passengers weren't wearing seatbelts.

"There was no warning and then half of them, their head hit the roof all at once," he said. "And I went, 'Oh, this is a bad day."'

Babies and children were crying as crew members went through the cabin assessing injuries. About 15 minutes later, there was an announcement asking for passengers who are medical professionals to help, Beam said.

The turbulence happened at 10,973 metres about 966 kilometres southwest of Honolulu, said US Federal Aviation Administration spokesman Ian Gregor.

The Boeing 777-200 was carrying 269 passengers and 15 crew members, according to Air Canada spokesman Peter Fitzpatrick.

Air Canada was arranging hotel accommodations and meals in Honolulu and options for resuming the flight.

"If we're going to be stuck somewhere, I can think of worse places," said Beam, travelling with her 10- and 11-year-old children.