Tragic new details emerge after child is killed and two others injured during mum's 'picnic' outing at Valley Heights, in the Blue Mountains

A father who believed his wife had taken their three young children on a picnic received a call at work telling him his five-year-old son was dead and the rest of his family was fighting for life.

Police allege the 42-year-old mother stabbed all three children, aged five and under, on a quiet street in the Blue Mountains on Sunday afternoon before turning the knife on herself.

The five-year-old boy died, while his two younger siblings were critically injured.

The children were strapped into their car seats when the alleged attack unfolded in Valley Heights.

The family was discovered after two cyclists and a passing motorist stopped on Waratah Road and desperately tried to help.

The woman and her two surviving children underwent surgery at Westmead Hospital and The Children's Hospital at Westmead and remained in a critical condition on Tuesday.

Detectives plan to speak with the mother if she recovers from her injuries, with a homicide charge expected to be considered.

The children's father rushed to hospital to be with his two surviving children after being told of the tragedy.

A father thought his wife had taken their three young children on a picnic. Instead, while he was at work, he received a devastating call telling him his five-year-old son was dead and the rest of his family was fighting for life 

Police allege the 42-year-old mother stabbed all three children, aged five and under, while they were strapped into their car seats in Valley Heights, before turning the weapon on herself

NSW Police Deputy Commissioner Peter McKenna described the scene confronting officers as 'barbaric'.

'When you're using knives, or edged weapons ... it's actually quite barbaric,' McKenna said.

'People cut people, it's actually very confronting.

'Whenever there are children, it doesn't get worse than that.'

The father is receiving support from Sydney's Kenyan community, while the Kenyan consulate has confirmed it is helping the family.

Local resident Julie Sukkar initially believed the commotion just metres from her front door was the aftermath of a traffic accident.

But when she went outside, she saw a white four-wheel drive parked on a vacant block with its doors open and the children inside, while their mother lay nearby in the bushes.

'One of the children, a little girl, a baby, had a beautiful pink and white flower dress on,' Ms Sukkar told the Daily Mail. 'I noticed that as they pulled her out of her car seat.'

Local resident, Julie Sukkar, initially believed the commotion just metres from her front door was the aftermath of a traffic accident

Two cyclists and a passing motorist stopped on Waratah Road and desperately tried to help. A witness said the children were eerily silent inside the car while their badly injured mother screamed and thrashed in nearby bushes 

'I only thought there were two children at first, but there were three of them in there.

'The woman on the ground was yelling, screaming and thrashing about. I'm not sure what she was saying, or if she was trying to say anything.

'She was covered in blood and had some really bad injuries.'

But amid the frantic activity around the car, it was the children's silence that Ms Sukkar found particularly difficult to witness.

'The children were quiet. They were not screaming or crying. They were not moving.

'That was hard to see. The police and the ambulance were here fast, and they did such a good job.'

Ms Sukkar said police initially handcuffed the mother before later removing the restraints as she was stretchered into an ambulance.

'They took them off pretty quickly, once they realised she was no danger to anybody,' she said.

The five-year-old boy died, while his two younger siblings and their mother remained critically injured in hospital. Their father rushed to be with his surviving children as Sydney’s Kenyan community rallied around the devastated family

Police said the family has no links to the Blue Mountains and were not previously known to officers.

Police will allege the wounds sustained by the mother were self-inflicted.

Ms Sukkar also praised the quick actions of the three strangers who stopped and raised the alarm, saying their decision to help may have saved the lives of the two surviving children.

'There were two cyclists and one woman who I think was driving past and stopped,' she said.

'I think those kids probably wouldn't have survived if they hadn't stopped.'

Despite witnessing the bloody aftermath and later learning that police suspect the mother was responsible, Ms Sukkar said she could not bring herself to condemn her.

'I felt really sad for her and for the kids as well,' she said.

'To be in that state of mind, I couldn't even imagine it.

'I have read Facebook posts with people calling her a monster. I think people can point the finger a little too early before they know what's happened.

'If you haven't been through mental illness, you wouldn't understand it.'

If this story affected you, call Lifeline on 13 11 14 or Beyond Blue on 1300 22 4636.