A father who fatally shot his two teenage children in Sydney's northwest before shooting himself had been involved in a long-running custody battle and “planned” the shooting, police say.
The 68-year-old gunman's body was found in a Normanhurst home, 5km from the scene of the initial killings, about 12 hours after his two children were found murdered, NSW Police confirmed on Friday.
A 15-year-old boy and a 13-year-old girl were found dead in their bedrooms at a West Pennant Hills house shortly before 5.30pm on Thursday.
The 68-year-old man was the father of the children but did not live at the West Pennant Hills home, police said.
Two handguns were found at the Normanhurst property. Police have confirmed they were registered in his name
Police say they were "aware" of the father.
The teenagers had been at home by themselves, police say. Their mother also lived at the home with the children but was not home at the time.
"I do know there were some custody hearings over the last two years."
The mother was treated by paramedics for severe shock and has been taken into care by friends.
Det Supt McFadden has described the crime scene at the Hull Rd home as "absolutely horrific".
NSW Ambulance Inspector Kevin Sweeney said paramedics were faced with an "extremely distressing and stressful situation".
Police said the murder was “premeditated and planned.”