The Sydney father who shot dead his two teenage children has been identified.

A Sydney father who planned the horrific murder of his two teenage children before taking his own life had been involved in a long-running custody battle, police say.

The body of financial planner John Edwards, 68, was found early on Friday in his Normanhurst home, roughly 4km from where he killed his 15-year-old son and 13-year-old daughter in their West Pennant Hills home about 5.20pm on Thursday.

Police believe the gunman took his own life very soon after the shooting, which was overheard by neighbours.

“The information we’ve gleaned … leads me to believe this was premeditated and planned,” Acting Assistant Commissioner Brett McFadden told reporters in Sydney on Friday.

“It’s too early to get a full appreciation of what went on in the (West Pennant Hills) premises.”

Mr McFadden described the crime scene at the Hull Rd home as “absolutely horrific”.

West Pennant Hills: Two young teenagers, a boy and a girl, shot dead in their bedrooms. Their father 68-year old John Ian Edwards was found dead in Normanhurst after the incident. @dantececcon #7News pic.twitter.com/66MDL4Ae6y

— 7 News Sydney (@7NewsSydney) July 6, 2018

Two “quite powerful” handguns, registered in the father’s name sometime this year and used to kill the children, were found at the Normanhurst home, he said.

“A number” of other guns owned by Mr Edwards were also found.

Police say they were “aware” of Mr Edwards, who had been involved in a custody battle with the children’s 36-year-old mother for two years following the breakdown of a long-term relationship.

But the exact nature of that relationship and any violent incidents between the pair were still being investigated.

The mother, who arrived at the house shortly after the Thursday afternoon shooting, was treated by paramedics for severe shock before being taken into the care of friends and police.

“We will be extending that support through additional counselling services today,” Mr McFadden said.

NSW Ambulance Inspector Kevin Sweeney said paramedics were faced with an “extremely distressing and stressful situation”.

1800 RESPECT (1800 737 732)

Lifeline 13 11 14

Kids Helpline 1800 55 1800 kidshelpline.com.au