It is understood the truck driver was not aware the woman was trapped underneath.
The woman was crossing the road near the intersection of Pittwater and Old Pittwater roads when she was struck.
She died at the scene.
The driver of the truck, a 57-year-old man, was uninjured and taken to hospital for mandatory testing, police said.
All northbound lanes of Pittwater Rd were closed and motorists were being diverted via Old Pittwater Rd, while southbound traffic was also affected and motorists were being advised to use Warringah Rd instead of continuing south through Brookvale.
Police then set in place a contraflow arrangement, opening one northbound lane and two southbound lanes, although traffic remained heavy.
Police from the Crash Investigation Unit and Northern Beaches local area command were examining the crash scene to determine the cause of the tragedy and police will prepare a report for the Coroner.