Sydney boy's truck escape 'extraordinary'

Police have described as "incredibly lucky" a four-year-old boy's escape from a truck accident that killed his mother on Sydney's north shore.

The 10-tonne vehicle rolled backwards down a driveway on the Wahroonga property of Priya Muthu on Wednesday, then tipped over and hit her.

The 30-year-old was trapped under the tipper truck and died at the scene.

Ms Muthu was holding her four-year-old son, who escaped with minor injuries.

Police say it's extraordinary the child was not seriously hurt.

"It's incredibly lucky he wasn't injured as well, and you can only be thankful this incident wasn't worse than it actually is," Superintendent David Jones told reporters.

Ms Muthu was overseeing construction at the property, where she did not reside, and the truck was removing soil and rocks from the backyard.

Investigators are trying to work out how the truck rolled backwards, and a mechanical examination will clarify whether the handbrake was on or not.

The truck's 67-year-old driver was adjusting the side mirror when the tragedy happened.

"At this stage the man is assisting us with our inquiries," Supt Jones said.

"He is obviously very distressed and shaken by what has happened."

Supt Jones said if negligence was found to be a factor, charges were possible.

The death of Ms Muthu, who worked as a database administrator at Wesley Mission, has shocked her family and colleagues.

"Priya was a well-respected and loved member of our information services team, who like us all are devastated by the news of her death," Wesley Mission chief executive Reverend Dr Keith Garner said.

"Her husband and their little boy are in our prayers at this time."

Ms Muthu's son is in the care of his father and family.