Everything we know so far about the 'horrific' West Pennant Hills murder.

With AAP.

Detectives are trying to piece together what happened inside a Sydney home before a father shot his two teenaged children dead in a “planned and premeditated” attack.

John Edwards’ 15-year-old son and 13-year-old daughter were killed in a bedroom at their Hull Road home at West Pennant Hills about 5.20pm on Thursday.

About 12 hours later Mr Edwards was found dead in his own home at Normanhurst.

Police believe the 68-year-old took his own life soon after the shooting.

According to The Daily Telegraph, Edwards hired a car so that when he pulled up outside the family home, his children wouldn’t know it was him. He timed the attack just before school holidays, so he knew his son and daughter would be at home alone without their mother.

When Olga Edwards arrived home on Thursday at around 6.30pm, it’s reported she collapsed from “severe shock” as she saw the “horrific scene” that lay before her.

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

People who know the family told The Daily Telegraph Edwards was anti-social, and a loner.

Acting Assistant Commissioner Brett McFadden said it was too early to say exactly what happened in the West Pennant Hills house but police believe the killings were “planned and premeditated”.

They were aware of Mr Edwards over past matters but he had no recent issues with the law.

He had been involved in a custody battle with the children’s 36-year-old mother, Olga, for two years following the breakdown of their marriage.

But the nature of their relationship and any violent incidents between them is still being investigated.

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, Mr McFadden added.

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

Mr McFadden said the mother, who was treated by paramedics for severe shock, has now been taken into the care of friends and police.

Mr Edwards, described as a “recluse” by a neighbour, was a financial planner and volunteered with the NSW Rural Fire Service, Lions Club and Pennant Hills Junior AFL Club, according to his LinkedIn page.

Juliette Hackett, who lives near Mr Edwards’ home, said she didn’t know the family well but the teenagers “had their challenges”.