Acting Assistant Commissioner Brett McFadden told reporters it appeared to be a "targeted incident" related to domestic violence. He said the victims were found dead by officers called to the "horrific scene" following reports of gunshots.
"It was a very harrowing experience for the emergency services who responded," he said.
"You've got a family that is going to struggle to come to terms with it and emergency responders will struggle to come to terms with what they have seen."
The victims' mother, aged in her 30s, arrived at the scene after police and was treated for severe shock.
Acting Assistant Commissioner McFadden told news.com.au it had not yet been determined how many shots were fired but that police were speaking with several witnesses.
A man known to the family was Thursday night being sought as a "person on interest".
"At this stage, the person of interest we are looking for is still at large but we believe this to be a targeted incident and there are no risks to the community at this time," Acting Assistant Commissioner McFadden said.
He told reporters the attack was a "very harrowing experience for those initial responding emergency service providers".
He said paramedics, who were visibly shaken by what they had seen, were comforted by a NSW Ambulance chaplain.
"It is what can only be described as a tragic set of circumstances for the whole family," he added.
NSW Ambulance Inspector Kevin McSweeney told reporters that when paramedics arrived they "were confronted with an extremely distressing and stressful situation".
"Two teenagers were unfortunately deceased in their bedroom and unfortunately at that time there was nothing we could do to help them," he said.
The mother was being treated by paramedics at the scene more than two hours after the fatal incident, but was later with friends away from the crime scene.
The bloody attack took place in the house on Hull Road, just off the Cumberland Highway.
At the scene on Thursday evening, forensic investigators wearing white protective suits could be seen walking up and down the street with torches and cameras.
About 150m of the road was blocked off, with witches hats stopping cars from entering one end, and uniformed police officers standing guard on both. Some residents who lived in the exclusion zone were escorted back inside their houses upon arriving late to the horror that, on this occasion, had come too close to home.
Longtime resident John Howard, 82, told news.com.au it was "normally a pretty quiet street".
Howard said he didn't hear any gunshots. He was alerted to the incident when his daughter saw it on the news and contacted him before he came outside.
"I couldn't believe it," he said. "It's so horrific … especially for such young people to be the ones killed."
About 20 residents, including young children, gathered on the street while police examined the crime scene. Homicide detectives continued doorknocking neighbours late into the night in search of information as to exactly what transpired. For now, all that is certain, is that two young people who were alive in their homes just hours ago — are now dead.
The double murder was just 5km from Carlingford, where a five-year-old was allegedly stabbed to death by his 36-year-old father last month.
