A grandmother pushing her grandson in a pram has been crushed and killed by a garbage truck on Sydney's northern beaches, but the toddler has escaped unharmed.

The woman, believed to be in her 50s, was on Pittwater Road in Dee Why when she was struck around 11.45am on Thursday, police said.

The young boy, aged about two years old, located in a pram nearby, was not injured during the incident.

Police said the woman died at the scene.

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"The family is very distressed understandably," NSW police Inspector Ellen Kaserman told reporters in Dee Why on Thursday.

"Fortunately the infant was not injured and has been reunited with his parents," she said.

A Careflight helicopter had to make an emergency landing on the busy road and paramedics treated the woman, who had sustained critical injuries.

She died at the scene, police said.

The truck driver was "traumatised and very upset" and was co-operating with police, Insp Kaserman said.

He was taken to hospital for mandatory blood and urine tests.