A Qantas flight attendant broke her leg and another suffered numerous injuries when their aircraft's autopilot unexpectedly disengaged as it descended into Canberra Airport last year.

The Australian Transport Safety Bureau's investigation into the incident has found a modification made to the Boeing 737-8 meant the pilot's attempt to stop the aircraft going too fast inadvertently triggered an abrupt change in speed and angle.

The Qantas 737 had been modified so autopilot cut out, instead of reverting to another mode. Credit:Craig Abraham

The incident occurred as the flight from Perth was preparing to land in Canberra about 10pm on March 13 when it encountered light to moderate turbulence, the ATSB's final report released on Wednesday found.

The "fasten seat belt" sign was illuminated and cabin crew moved towards their seats at the front and rear of the plane.