Police have alleged that a mother was responsible for a stabbing attack involving her three young children, leaving one child dead and two others critically injured in hospital.

The woman, who is in her 40s, is also fighting for her life in critical condition.

A group of cyclists discovered the woman and three young children suffering stab wounds on Waratah Road in Valley Heights, near Springwood, shortly after midday on Sunday.

Despite witnesses attempting to help and providing first aid before emergency crews arrived, one child died at the scene.

The children were found inside a car that had partly entered a vacant block beside a fire trail leading into the national park. The woman was located outside the vehicle nearby.

Waratah Road resident Peita Davies said neighbours knew one another and did not recognise the woman or children as locals.

“Nobody heard anything,” she told the Sydney Morning Herald. “We all want to know how they ended up here.”

Local resident Glenn Lyall initially believed the large police operation was connected to a serious crash.

“I could see a hatchback on an angle and two cyclists pointing, and I thought the car had hit one of the cyclists,” Lyall said.

Another resident described the scene as horrifying, saying there was a large amount of blood.

The alleged domestic violence incident came at the end of a devastating weekend in NSW, after three women were allegedly murdered in their homes over a 12-hour period on Saturday.

NSW Police Deputy Commissioner Peter McKenna described the preceding two days as devastating.

“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 the no-more-tragic situation than what we had in the Blue Mountains, put them all together and it’s unfathomable.”

Paramedics treated the three at the scene on Sunday before transporting them by road to Westmead Hospital and Westmead Children’s Hospital in critical condition.

“The two children and their mother are in a critical condition,” Mr McKenna said.

‘The mother is the suspect in this matter, for domestic related homicide, and that investigation is ongoing.’

Hours after the incident, NSW Police Superintendent John Nelson said the father of the children had been notified.

He said the father had been at work when he received the call from police.

‘You can imagine receiving that tragic news,’ Supt Nelson told reporters.

He said the woman had not previously come to the attention of police and that investigators were not looking for anyone else in relation to the incident.

The woman is aged in her 40s, and the children are all under five years old.

All three have undergone surgery in hospital.

Supt Nelson praised the efforts of the ‘upstanding’ members of the public who responded to the tragic scene.

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

He said responding officers would not be spared the hardship and would have access to available support services.

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

Blue Mountains councillor Darren Rodrigo on Sunday described the news as ‘absolutely horrendous and tragic’.

‘Our deepest love and condolences to the immediate family and our prayers with all the children involved,’ he said.

‘A massive thank you to the community members (who) intervened to provide medical assistance and to our first responders for their incredible support and courage during this time.’