An eight-year-old girl has died after a tree branch fell on her in her primary school playground at lunchtime.
Traumatised children have left the northwest Sydney school in the arms of their parents after their schoolmate died in hospital, where two other children and a teacher are in a stable condition.
Emergency services were called to Pitt Town Public School about 1pm on Friday after the gum tree branch fell on the three children and teacher.
The grade four girl went into cardiac arrest as she was pinned under the branch.
She was rushed to Westmead Children's Hospital in a critical condition but died shortly after.
The teacher in his 50s suffered head injuries and a five-year-old kindergarten student and her seven-year-old brother were also hurt.
The children are in a stable condition in hospital.
The teacher is also in a stable condition and resting, a Westmead Hospital spokeswoman said.
Parents flocked to the Sydney school to pick up their children, some of whom saw the tragedy unfold.
"I came around and the tree was where I was sitting for lunch and we were just rushed to the other side of the school," a pupil told Network Seven.
Another child said: "I heard a big crack go down on the teacher and five other girls."
Police said an arborist estimated the gum tree was about 50 years old and weather conditions did not contribute to the fall.
"The school and department of education are going to take action in relation to the tree, getting it further inspected over the weekend," police inspector Bill Slatford told reporters.
Hawkesbury Mayor Kim Ford said he would talk with the family "when the time's right".
"It is a tragic event and by all accounts it's just a freak accident that's happened," he told AAP.
NSW Education Minister Adrian Piccoli said police were investigating the tragedy.
"This is a deeply distressing time for family and friends of the child and I extend my deepest sympathies to them," he said in a statement.
"Counselling is being offered to students and staff."