The 68-year-old man found dead this morning after fatally shooting his two teenage children at a Sydney home has been identified by police.

The father, financial services worker John Edwards, had been involved in custody hearings about the children over the past two years.

In what police described as a "planned attack", police say Edwards went to a West Pennant Hills home and shot his son Jack, 15, and daughter Jennifer, 13, shortly before 5:20pm on Thursday.

They were found in their bedrooms in the home they shared with their mother.

After an overnight search, police discovered Edwards' body this morning at his home in Normanhurst, a suburb on Sydney's Upper North Shore, about 5 kilometres from where the children were killed.

Two guns, described as "powerful handguns" by police, were found at the property.

Sorry, this video has expired Police were called to a house on Hull Road in West Pennant Hills about 5.20pm

"The information we've gleaned in the last 15 hours [leads] me to believe that this is something premeditated and planned," NSW Police Acting Assistant Commissioner Brett McFadden said.

"And we believe that shortly thereafter he went to the Normanhurst address and took his own life."

The children's 36-year-old mother returned to her home shortly after police arrived on Thursday night.

Police believe the teens were at home alone when their father shot them.

"[The mother] attended the scene suffering from significant shock. I can't even imagine the stress and the heartache that she is going through right now," Commissioner McFadden said.

"It is a horrific homicide … this is a most traumatic incident."

Edwards was known to police, however Commissioner McFadden said he had no "contemporary" criminal record.

Police said there had been custody hearings over the two children.

"The details about the nature of the relationship, particularly in recent times and the level of access that the 68-year-old had to his children is subject to investigation," Commissioner McFadden said.

Posting on Facebook, the student leadership council at Pennant Hills High School, where one of the teenagers attended, said their "hearts broke" on finding out the news.

"Today [we] found out that one of our students was taken from us too soon," the statement said.

"Our deepest condolences go out to any students, friends, family and teachers that are affected by such heart-breaking circumstances, and such a devastating event."

The children were shot dead at their West Pennant Hills home. ( AAP: Dan Himbrechts )

'Nothing can prepare you'

Floral tributes are left outside the home. ( ABC News: Mark Reddie )

Commissioner McFadden described it as "heinous crime".

"When you prepare yourself as a police officer you prepare yourself to deal with the most tragic of circumstances," he said.

"But nothing can prepare you to walk into a home and see two children in that state. I managed to speak to some of the police involved. They had a role to play and they did it well."

Speaking on Thursday night, NSW Ambulance Inspector Kevin McSweeney said when paramedics arrived at the scene they were confronted with "an extremely distressing and stressful situation".

He said there was nothing that could be done for the boy and girl.

The family and emergency services personnel who attended the scene are being given support.

"You've really got a family that's going to be struggling to come to terms with it but also the responding police will struggle to come to terms with what they've seen."

'It's so cruel, it shouldn't happen'

Police set up a road block at Dean Street and Hull Road in West Pennant Hills on Thursday night where a group of concerned neighbours gathered.

Forensics at the scene in West Pennant Hills in Sydney's north-west this morning. ( AAP: Jeremy Piper )

One neighbour, Maya Clarke, said she was "shocked and scared" to hear of the tragedy.

"It's just a nice place, a nice area," she said.

"It should not happen … it's so cruel, it shouldn't happen.

"How people can shoot kids … I'm shaking."

Her granddaughter Sienna Clarke said it was a quiet and peaceful area.

"Bad things don't happen around here so it's just kind of frightening because our house is just down the street," she said.

John Howard, another resident, has lived in the area since 1963 and said there was very little crime.

"It is horrendous and it's just something that we never really worry about in this area. We feel very safe."