Air Canada Flight 34 lands at Vancouver International Airport Friday morning, after making an unscheduled stop in Honolulu. (CBC)

At least 11 passengers and four crew members were injured early Friday when Air Canada Flight 34 from Australia to Vancouver hit severe turbulence caused by a thunderstorm about one hour northeast of Honolulu.

Passengers said the plane suddenly dropped, throwing them up against the ceiling. The seatbelt sign was on when the plane hit the turbulence, according to the airline.

Lindsay Hamon, who was sitting in business class, said passengers not wearing seatbelts suffered the most.

"There was about 15 minutes of really violent turbulence and anyone who wasn't strapped in was in trouble, and they took a number of passengers off at Hawaii. Some of them came back on. They had to stretcher maybe four out of the airplane," he said.

Hamon said while the incident was frightening, most passengers remained calm, and there were a number of doctors on board who helped treat the injured.

After the incident, the pilot turned the Boeing 777 around and made an unscheduled stop in Honolulu around 1 a.m. HT Friday, Air Canada spokesman Peter Fitzpatrick said.

Upon landing in Honolulu, two passengers and two crew members were taken to hospital, while nine other passengers and two other crew members received medical care at the airport and were released to continue their journey.

The plane departed with the remaining passengers and crew after about two hours, and landed in Vancouver shortly before noon PT.

The passengers and crew left in Honolulu sustained minor injuries and have since been released from hospital.

The crew is believed to be based in Vancouver. The passenger list indicated the Boeing 777-200LR was originally carrying 256 passengers and 17 crew members.