A 15-year-old boy is under arrest after another teenager was fatally stabbed behind a high school.
Police say two boys were approached by two or three other young males in a park in Sydney's northwest about 4.20pm on Monday when a 17-year-old boy was stabbed in the thigh.
He was treated by NSW Ambulance paramedics but was unable to be saved and died at the scene, at Bruce Purser Reserve behind Rouse Hill High School.
Police said those involved were not from the school and officers believe it was not a random attack.
Detective Superintendent Naomi Moore said the 15-year-old boy had presented to a police station with their parent and was under arrest and assisting officers.
Det Supt Moore said a number of female witnesses who were in the park at the time had given aid to the victim.
"It would have been a horrifically confronting situation for those people and I can only praise them for their efforts," she told reporters.
"It was a potentially a dangerous situation they were walking into, but they thought nothing of themselves to go in and render aid."
Det Supt Moore called the teenager's death a senseless tragedy.
"It's a tragic situation. I have 17 -year-olds at home. This has impacted the core of the community. I have no other words, but it's just tragedy and it's senseless."
NSW Premier Chris Minns sent his condolences to the teenage boy's family and friends.
"I am deeply saddened by the death of a teenage boy at Rouse Hill and extend my sincere condolences to his family and friends," he said in a statement.
"It is an unimaginable loss, and the whole community will feel the impact."
Federal Attorney-General Michelle Rowland, whose electorate of Greenway includes Rouse Hill, said she was deeply saddened to learn of the loss of such a young life.
"There is no place in our community for these acts of senseless violence, and I know this news will be particularly distressing for local families," she said in a post to Facebook on Monday evening.