Rex propeller under analysis in Sydney

Investigators are poring over a damaged 100-kilogram propeller to figure out how it came off a Regional Express plane mid-flight over Sydney last week.

After coming off the Albury to Sydney flight on Friday, the propeller was finally found on Tuesday in a national park in Revesby, 18 kilometres from Sydney Airport.

It was winched out of the bushland on Wednesday by a helicopter before being loaded onto a truck and taken to an aviation facility in Bankstown for analysis.

Australian Transport Safety Bureau investigators will now try to determine exactly what caused it to separate from Rex's Saab 340.

ATSB chief commissioner Greg Hood said getting to the bottom of the separation will help prevent a recurrence.

But Australia had not seen anything like this event before.

"This is a first - a very rare occurrence."

The only other recorded similar event was in 1991 when US carrier Comair's aircraft also landed safely after losing a propeller.

Regional Express has temporarily grounded five planes "through an abundance of caution" while it replaces gear boxes and shafts from the same series as the propeller that detached.