A crack in a crucial mounting bracket was the reason a propeller fell off a Regional Express plane last month, the Australian Transport Safety Bureau has found.

The ATSB released their preliminary report into Flight ZL768, which was forced to make an emergency landing after the right propeller fell off mid air.

The propeller was found a few days later in bushland off the River Road in Revesby in Sydney's south-west.

The ATSB stated a crack had formed in the propeller mounting flange, where the propeller attaches to the gearbox, and moved through to the shaft.

There was corrosion in a dowel pin bore, where the crack originated, and the ATSB is still investigating whether this contributed to the separation of the propeller.

The report found corrosion or cracking of this kind may not be seen until it reaches the surface, as there are currently no routine inspections required of dowel pin bores.

The cracked bracket that the ATSB says caused the propeller to fall off a REX flight. ( Supplied: ATSB )

ATSB chief commissioner Greg Hood said Regional Express quarantined all propeller gearboxes with the same series of shafts that was in the affected aircraft.

"This is the first known critical failure of this type, initiating within the propeller hub flange of the GE Aviation CT7-9B engine," Mr Hood said.

"Both the operator and the engine manufacturer have already taken proactive safety action in response to the ATSB's safety advisory notice.

"GE Aviation is conducting metallurgical analysis on the propeller flange and hardware at its laboratories in Cincinnati, USA. They are also inspecting additional propeller gearboxes and reminding all operators to follow maintenance and inspection procedures.

"GE Aviation will release additional maintenance and inspection recommendations if they become necessary."