A FACS carer opened her car door while parked in a NSW motorway breakdown bay and a boy in her care darted into traffic. She followed him in front of a truck.
Police believe an eight-year-old boy in state care and his 27-year-old female carer were parked in a breakdown bay on the M1 motorway south of Newcastle when the boy darted into traffic.
The woman gave chase - and the pair was killed when they ended up in the path of a truck which was unable to stop in time on Sunday about 5.30pm.
The carer, named by News Corp Australia as Rachel Martin, had just opened the car door when it appears the boy ran out, police said on Monday.
The boy, from Woy Woy, was in the care of Family and Community Services (FACS) while Ms Martin, from Cooranbong, was employed by the department, AAP understands.
Paramedics and emergency services shut down the M1's southbound lanes for nearly six hours as the police crash unit launched an investigation.
The truck driver, a 57-year-old man, was treated for shock before being taken to John Hunter Hospital for blood and urine testing.
A report will be prepared for the coroner.
"The death of any child is a tragedy," a FACS spokeswoman told AAP on Monday, adding the department was deeply saddened to hear of the tragedy.
"As this is currently a police investigation, FACS is unable to comment further."
Highway Patrol Acting Assistant Commissioner Stuart Smith said the double fatality capped of a "weekend of tragedy" in which nine lives were lost on NSW roads.
"The driver is taking it fairly hard," Commissioner Smith told reporters in Sydney on Monday.
"There are different levels of grief after yesterday's exposure," he added, noting counsellors were on scene for emergency workers.
In a separate incident, two men died near Deniliquin on Sunday evening when their ute rolled.
A 64-year-old man died earlier on Sunday when he crashed his motorbike while riding in the Tweed region.
In the NSW Southern Highlands, a passer-by discovered an overturned car early on Sunday morning, with the body of a 30-year-old man in it.
The weekend's nine deaths bring the state's road toll total to 332, five more than the same time last year, police said.