A young boy has died and two others are fighting for life after an alleged stabbing in the Blue Mountains, west of Sydney.

Emergency services were called to Waratah Rd in Valley Heights about 1pm on Sunday following reports a woman and three children had suffered stab wounds inside a vehicle.

WATCH THE VIDEO ABOVE: Tragedy in Blue Mountains as boy dies in attack

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7NEWS understands a woman riding a bike came across the vehicle and found the woman bleeding.

Members of the public rushed to help, providing first aid to the woman and children while contacting emergency services.

However, one of the children died inside the vehicle at the scene.

The other two children suffered wounds to their necks.

They and the woman were treated by paramedics before being rushed to The Children’s Hospital at Westmead and Westmead Hospital in critical conditions.

A major emergency response unfolded on Waratah Rd in Valley Heights on Sunday afternoon. Credit: 7NEWS

A child has died and two others are fighting for life after an alleged stabbing in the Blue Mountains. Credit: 7NEWS

While they are yet to be formally identified, all three children are believed to be aged under five, while the woman is believed to be in her 40s.

Superintendent John Nelson said the woman was not previously known to police.

The children’s father, who was at work when police contacted him, has been informed of the tragedy.

“We’ve made contact with the father and obviously you can imagine receiving that tragic news,” Nelson said.

“That person is at work just going about their business and they’ve had this information provided to them.”

Nelson described the incident as “traumatic and tragic” following a series of deadly domestic violence-related incidents across NSW.

“Let me just say it’s obviously been a very difficult weekend, a very distressing weekend for many people,” he said.

“This is the fourth death that we’ve had, potentially domestic violence-related.”

Superintendent John Nelson said the woman was not known to police. Credit: 7NEWS

Three women, aged 23, 70 and 90, died in separate alleged domestic violence incidents across NSW on Saturday.

Nelson added the wider family of the alleged Blue Mountains victims were yet to be informed.

A crime scene has been established as police investigate the circumstances surrounding the incident.

Police said they are not looking for anyone else in connection with the incident at this stage.

Anyone with information is urged to come forward.

If you or someone you know is impacted by sexual assault, domestic or family violence, call 1800RESPECT on 1800 737 732 or visit 1800RESPECT.org.au.

In an emergency, call 000.

Advice and counselling for men concerned about their use of family violence: Men’s Referral Service, 1300 766 491