NSW police believe the children, a boy, 15, and girl, 13, were killed around 5:20pm last night at their home, in the northwestern Sydney suburb of West Pennant Hills.
The mother is recovering after being treated for severe shock.
Brett McFadden, Acting Assistant Commissioner, claimed the incident was a 'targeted incident' related to domestic violence.
'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.'
'It is what can only be described as a tragic set of circumstances for the whole family,' he added.
After police issued a statewide manhunt, it's been confirmed that the suspect, aged in his 50s, has been found dead.
More details to come.
This article originally appeared on New Idea.