A child is dead and two others are fighting for their lives after they were discovered with stab wounds inside a car in a tiny Blue Mountains town.

Passing cyclists reportedly spotted the three children and a woman inside a vehicle on Waratah Road, at Valley Heights, shortly before 1pm on Sunday.

Bystanders gave first aid before paramedics arrived and treated the injured. They were then taken by road to Westmead and Westmead Children’s hospitals.

A five-year-old boy died at the scene.

NSW Police said on Monday that the alleged stabbing was being treated as a domestic violence-related incident.

It was among a string of “shocking” DV-related crimes across the weekend, including a grandson charged with the alleged murder of his grandmother, with his lawyer claiming he was affected by Ozempic.

Valentina Spivak, who was in her 90s, was found with significant stab wounds to her torso about 5.15pm on Saturday at Sydney’s Bondi Beach. She died at the scene.

Sergei Mitnik was arrested at the home before being charged with domestic violence-related murder.

Valentina Spivak died in a Bondi Beach unit. Photo: 9NEWS

Lawyer Javid Faiz said there were concerns about Mitnik’s mental health.

He noted an appointment with a psychologist was cancelled a month ago and Mitnik had been taking the medication Ozempic, which “had an effect on him”.

Hours before Spivak’s death, a 23-year-old woman died after being found with stab wounds inside a home in Wagga Wagga, in south-west NSW.

Adam Mark Perrim, 31, was arrested almost nine hours later at another home in the regional city. He was later charged with the woman’s murder.

Police said the pair were in a relationship.

Perrin appeared briefly in court on Sunday but did not apply for bail. He will next appear in court on November 18.

Later on Saturday, the body of a 70-year-old woman was found at Mole River, in northern NSW, after reports of a domestic violence incident about 9.35pm.

Police believe a 37-year-old man entered a property and tried to steal a vehicle before assaulting a 74-year-old male resident.

The younger man was arrested. When police went to search the neighbouring property, they discovered the woman’s body.

They said part of the house was damaged by fire.

The 37-year-old man has been charged with murder and is due to appear before court on Monday.

Police said he and the woman also knew each other.

Police and emergency services at the scene west of Sydney. Photo: AAP

Blue Mountains stabbing

Hours after the children and woman were found in the Blue Mountains, NSW Police Superintendent John Nelson said the father of all three children had been notified.

The father had been “going about [his] business” at work when he received the call from police, he said.

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

He said the woman was not previously known to police and investigators were not looking for anyone further in relation to the incident.

She was thought to be aged in her 40s. All the children were aged under five.

All three had surgery upon admission to hospital.

Nelson praised the efforts of the “upstanding” passersby 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.

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

“Like any human being, it’s a tragic circumstance to be exposed to,” Nelson said.

“They’re highly professional officers, and they responded in such a professional way.”

Homicide investigators and local police remained at the scene on Sunday afternoon, with part of the road closed to traffic.

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

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.”

Anyone with information about the stabbing or with footage of the attack is urged to contact police.

NSW Premier Chris Minns said his thoughts were with the families and communities.

“These are shocking crimes and families are today dealing with unimaginable grief,” he said.

NSW Opposition Leader Kellie Sloane said she was horrified by the three women’s deaths.

“My thoughts are with their families, friends and communities, and with the police, paramedics and neighbours who responded,” she said, adding the incidents could not be treated as “background noise”.

Almost a third of all homicides in Australia are family or domestic violence-related, according to Australian Bureau of Statistics data.

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beyondblue 1300 224 636

-with AAP