A 17-year-old girl is accused of stabbing her younger sister to death while they were alone in the family home, with police saying the "unthinkable" crime happened "completely out of the blue".
Emergency services were called to the home in Boolaroo, west of Newcastle in the NSW Hunter region, to reports of a stabbing about 3.45pm on Monday.
Paramedics treated the 10-year-old for multiple stab wounds to the chest but she died at the scene.
Her older sister was arrested and interviewed overnight before being charged with domestic-violence related murder and has been refused bail while she remains before a children's court.
Superintendent Tracy Chapman said there is no record of police being involved with the family previously "for matters such as this".
Investigators will speak with neighbours and other family members.
Detective Superintendent Danny Doherty was reluctant to detail the dynamics of the family but said police believe the alleged attack took place in the family home.
"They were the only children in the house," the veteran homicide detective told reporters on Tuesday.
"It is very hard to imagine, it is unthinkable that this could happen to a 10-year-old."
The attack appeared to have occurred "completely out of the blue", Det Supt Doherty said.
"My thoughts go out to the mother at home having lost a 10-year-old in a tragic loss a life, and now has another daughter facing court allegedly responsible for killing the 10 year old so this is terrible situation for her to be in," he said.
Police Deputy Commissioner of Regional NSW Field Operations Paul Pisanos said emergency responders tried to revive the girl.
“Emergency services did try for some time to try and revive the child, but unfortunately they were unsuccessful and the child was deceased,” he told Sydney radio 2GB.
Local MP Greg Piper said the tragedy had impacted two school communities and devastated everyone in the Lake Macquarie area.
"The family and the community will live with the implications of this for many years to come," he told AAP.
"Wherever I and my office can provide support, we will."
The girl will appear next at Broadmeadow Children's Court on June 24
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