Police were called to a home in Baulkham Hills at about 5.15am today.
There they found a 10-year-old boy, a 13-year-old girl, a 16-year-old girl and a 47-year-old woman, all with stab wounds.
The eldest child, a 16-year-old girl who is believed to use a wheelchair, was stabbed once and had a number of lacerations.
The 13-year-old girl had three stab wounds, and the 10-year-old boy had two wounds.
The 47-year-old woman, who police say is the mother of the children, had three stab wounds to the abdomen which officers believe were self-inflicted.
They were all treated at the scene and taken to hospital in a stable condition.
Detective Superintendent Naomi Moore told reporters this morning that a fifth person, believed to be the father of the children, secured a household knife allegedly used in the stabbings, and contacted police.
“We believe the father was in the home at the time,” Moore said.
“We believe the father and the children were all in their rooms and potentially all of them asleep when the mother has carried out this act.
“Upon waking up to what I would believe to be a number of screams or yells, he has then gone to approach the situation and from what I am told, has secured the weapon and he has contacted police.”
She said the family had no history of domestic violence and were not known to police.
All three children were believed to be in their rooms when the stabbing took place.
Police have not yet spoken to the 47-year-old woman, who is undergoing treatment, and no charges have yet been laid.
It is understood police are not looking for any other person.