Police believe the mother was behind a stabbing attack on three young children that left one dead and two others fighting for their lives in hospital.

The woman, aged in her 40s, is also in a critical condition in a Sydney hospital after Sunday’s disturbing incident.

Walkers who were first on the scene found the three children and the woman with stab wounds inside a parked vehicle in the Blue Mountains, west of Sydney, on Sunday afternoon.

One young child, a five-year-old boy, died at the scene despite witnesses rushing to help, and delivering first aid while waiting for emergency services to arrive.

NSW Police Deputy Commissioner Peter McKenna said the children were siblings.

“I can state that the female is the mother of all three children and is a suspect,” he said.

“We’re less than 24 hours into an investigation. I’m not aware of any motive of any incident as to why this occurred.”

McKenna said police would allege the mother’s injuries were self-inflicted.

“We have to wait for the medical side to play out. Once that has played out, if she recovers from those injuries, our detectives will look to speak to her with the intent on charging with homicide,” he said.

Paramedics treated the two surviving children and the mother at the scene before taking them to Westmead Hospital and Westmead Children’s Hospital, where they remain in a critical condition.

McKenna said the family was found in what appeared to be a “random” location. The woman was not previously known to police and investigations into the incident continued on Monday.

The father is supporting his two children in hospital.

Sunday’s alleged domestic violence incident came at the end of a horror weekend across NSW, after three women were allegedly murdered inside their homes within a 12-hour period on Saturday.

“We’ve had four incidents, one at Wagga, one at Tenterfield, one at Bondi and … terrible scenes at the Blue Mountains yesterday,” McKenna said.

“It’s actually very, very confronting.”

He said all four scenes were “horrific”.

“All four scenes were horrific. When you are using knives or edged weapons, in which all four matters, we will allege, it’s quite barbaric,” he said.

“People who stab people, who cut people, it’s actually very confronting, and whenever there are children involved, it doesn’t get worse than that.”

McKenna said it had been a devastating two days.

“It’s a horrific and tragic weekend and it’s like one that I don’t remember in my career,” he told Nine’s Today.

“When you balance out those three and plus the no-more-tragic situation than we had in the Blue Mountains. Put them all together and it’s unfathomable.”

On Sunday, NSW Police Superintendent John Nelson said the children’s father was at work when police called him to tell him what had happened.

“You can imagine receiving that tragic news,” he said.

“He’s supporting these two children that are in a pretty good condition at the moment. He’s assisting us with the investigation.”

Nelson praised the efforts of the “upstanding” bystanders who responded to Sunday’s tragic scene.

“People are just going about a lovely Sunday afternoon and being confronted with this is traumatic and tragic for anybody,” he said.

Responding officers would not be spared the hardship and would have access to available supports.

Police said the weapon used and the circumstances surrounding the incident would form part of investigations.

NSW Premier Chris Minns said the weekend’s events were confronting and incredibly violent.

“We can’t become numb to the pain or the violence that we see,” he said.

“It’s profoundly distressing, and I understand why the people of Sydney and NSW are horrified over the last 24 hours”.

State Opposition Leader Kellie Sloane said she was “sick and tired” of wake-up calls about domestic violence.

“I’ve had enough. We need to do something,” she said.

“This is something that is not just on the government. It is on all of us.

“There is a sense of betrayal here, that across our country we’re being let down, that we are losing good women, mothers, grandmothers, sisters.”

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-with AAP