It was over the Pacific Ocean when the plane experienced what an Air Canada spokesperson described as "un-forecasted and sudden turbulence".
"The lady in front of us, I don't think she had her seatbelt on and she hit the ceiling", passenger Sharon Thornton told CTV.
The passengers have suffered minor injuries, reported ANI.
Forced to turn around, the pilots made an emergency landing in Honolulu early this morning, paramedics tended to the almost three dozen injured passengers.
The Boeing 777 aircraft landed in Honolulu at 12:45 pm local time (6:45 pm CET). "But as a whole people seem to be OK; didn't seem to be any major injuries".
Alex Macdonald, from Brisbane, told Canadian broadcaster CBC News the passengers on board were "extremely shocked". "There was no warning and then half of them, their head hit the roof all at once", he said. "I think one of the stewards was badly injured when the trolley fell on top of them", Bailey said.
The turbulence happened at 10,973 metres about 966 kilometres southwest of Honolulu, said US Federal Aviation Administration spokesman Ian Gregor. Its spokesman said crew members had asked for medical personnel to meet the plane at the gate.
The airline is making hotel and meal arrangements for the passengers in Hawaii, and will organize options for passengers wishing to continue onward to Sydney.
The flight had originated from Toronto and was reportedly carrying 268 passengers and crew.