Passengers have told of their terror after a Sydney-bound flight hit severe turbulence near Hawaii.

Air Canada flight AC33 was diverted to Honolulu after passengers aboard the Boeing 777 were tossed from their seats.

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

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.”

Passenger Stephanie Beam said: “The plane just dropped. 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.”

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

There were 269 passengers and 15 crew aboard the flights

The plane landed in Honolulu at 12.45pm on Thursday (1.45am AEST Friday).

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.

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.

“We hit turbulence and we all hit the roof and everything fell down, and stuff … people went flying,” passenger Jess Smith told CBC News.

“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.

“But as a whole people seem to be OK; didn’t seem to be any major injuries.”

The turbulence happened at 10,000 metres about 966km southwest of Honolulu, US Federal Aviation Administration spokesman Ian Gregor said.

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.