A Toyota Kluger SUV, allegedly driven by 52-year-old woman Maha Al-Shennag, had ploughed into a classroom full of 24 pupils and their teacher, landing on and killing two young boys not long into their morning class within the weatherboard building.

And then, just after 9.30am, all hell broke loose.

Khaled Arnaout, who had only returned home across Banksia Road moments before hearing the bang, said the screams kicked him and other would-be rescuers into gear.

"My daughter goes to that school so instincts told me 'my daughter'. So I ran across the road screaming 'my daughter' and I get there [to see a] big hole in the building," Mr Arnaout said.

"Teachers and everyone were just running around. There was blood and kids on the floor, just lying down and screaming."

Mr Arnaout and at least three tradesmen, who were already working on site, teamed up to lift the nearly two-tonne vehicle off several of the children.

The two boys, both aged eight, were unconscious after suffering critical injuries.

"I looked at the car and someone said, 'they are stuck under the car, help us, help us'. We lifted up the car straight away and just got him," Mr Arnaout said.

"He was a critical one, we took him to a room. We took them all inside to the office and straight away got him into CPR and put another kid in a recovery position.

"He was pretty bad, he had a big gash on his head. I kept doing CPR until [the ambulance] got there and then they took over."

Mr Arnaout said he could see the driver, who had been able to clamber out of the wreck physically uninjured, was in shock.

"I actually felt sorry for her but I couldn't give her any attention. I just left her there," he said.

The boys and three other children, two girls aged eight and a third girl aged nine, were rushed to The Children's Hospital at Westmead.

But the two boys could not be revived. One of the girls remained in a serious condition on Tuesday afternoon.

Their families were rushed to hospital only to be told the tragic news as waves of relatives descended on the school's two entrances.

Paramedics assessed 17 other children and their female teacher before they were placed back into the care of immediate family.

Acting traffic and highway commander Superintendent Stuart Smith said early investigations suggested the incident was not deliberate.

"We're not looking at this as an intentional act. It is a crash investigation," he said.

"We're working with forensic services group, we're methodically working through the circumstances of this crash. We have a 52-year-old local woman who is with police at this time. She's co-operating with blood and urine testing and the processes that accompany that investigation."

"This is a horrific, you know, this is a horrific incident for all."

Year four student Mariam Issmail was in a nearby classroom and said everyone in the class was very scared.

"We were just doing our work and all of a sudden we just heard this big bang," she said.

"We were crying. Kids were screaming and saying, 'I want my mum, I want people, I want help'.

"We had to stay in the classroom and stay until our parents came."

Mariam's mother Khadije said she was shocked upon hearing the news.

"My neighbour told me," she said. "I couldn't even speak."

Ambulance Service of NSW Superintendent Stephanie Radnidge said paramedics arrived to "a scene of carnage".

"[There were] a number of distressed and overwhelmed children and teachers at a scene of a horrible accident," she said.

"Obviously they were crying, they were distressed, some were asking for their parents, that's a natural response in such a terrible set of circumstances.

"It is very, very hard because we are parents ourselves, we are human beings.

"To those persons who have assisted in this terrible tragedy prior to paramedics arriving, of course the families, paramedics and everyone involved want to say thank you for going above and beyond for caring to those people in that terrible time of adversity."

Crash investigators were still attempting to discover what may have caused the vehicle to gather speed and career into the building from a small carpark within the school grounds.

The alleged driver was taken to hospital for mandatory drug and alcohol testing. She was charged late on Tuesday with two counts of dangerous driving occasioning death and bailed to appear in court later this month.

The Education Department said in a statement that the classroom was a permanent weatherboard building of sound structural condition. "While asbestos containing materials have been identified inside the building, the removal of any asbestos that may have been disturbed in the damaged building will be managed in compliance with SafeWork NSW requirements," a spokesman said.

Education Minister Rob Stokes said his thoughts were with the school community.

"I cannot begin to comprehend the sadness being felt by the parents and families of the two young boys who have died in today's tragic accident at Banksia Road Public School. My thoughts and prayers are with their families and the entire school community," he said in a statement.

"Our first priority is to support students, staff and parents by any means necessary and allow them time to properly grieve. This will not be a quick process and we will work together with the school community to provide whatever assistance we can.

"Thank you to the school staff and emergency services personnel for their commitment and fortitude in very difficult circumstances today. I thank them for all their best efforts and quick action as they worked to save lives."

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