Two years to the day since the terrible accident, the NSW Coroner's Court in Sydney was told on Tuesday that Myley Maxwell had been a "beautiful, healthy, happy little girl" and a "vibrant child".

Myley had gone to a birthday sleepover at a friend's house at Gwabegar, but many other details, such as who was there and the location of the death, cannot be reported for legal reasons.

"What were the circumstances leading up to the tragic accident on a quad bike, which caused her to lose her life?" the counsel assisting the coroner said in their opening address.

The court heard five children were playing at the farm on the morning of March 5 and were "given permission" to ride on two red adult-sized quad bikes.

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"The circumstances in which they came to be given permission, and what instruction or advice they were given by [names redacted] before they departed, will be the subject of evidence at this inquest," the counsel assisting said.

The children had set off from the house and, "regardless of what directions they were given, none of the children was wearing a helmet," the court heard on Tuesday.

The counsel assisting said the children had ridden the quad bikes around that property and another, and filmed their ride on a phone. 

The court heard Myley was a front passenger on one quad bike carrying three children.

It was travelling on a "well-worn dirt path" when it came off the path.

"The quad bike then collided with a small tree, which considerably slowed the quad down, before it collided with a second, much larger tree to the right-hand side," counsel assisting said.

All three children were thrown from the quad bike. One of them suffered minor injuries, another suffered a fracture to their skull and Myley "suffered a large impact to the right side of the skull".

The other children used a mobile phone to call a mother, and triple zero at 12.26pm. Family members went rushing to help the injured children, who were taken to a homestead, and CPR was performed.

The ambulance arrived at 1.02pm, but Myley was pronounced dead at the scene. Police and Myley's parents arrived and were "distraught, shocked and inconsolable with grief".

"It is impossible to imagine how they must have felt," the counsel assisting said.

Myley suffered non-survivable head trauma with multiple skull fractures and bleeding on the brain, the court heard. A second child was airlifted with a skull fracture.

None of the children involved are expected to give evidence at the inquest "because of the inevitable trauma and mental stress caused" after experiencing "this tragedy".

"There is no suggestion that children were driving in any way that was deliberately reckless," the opening address heard, adding some of them had experience riding quad bikes.

"But they are children, they were having fun and quad bikes can travel at speed."

The inquest is expected to hear evidence all four tyres on the quad bike were deflated, which could have contributed to the accident.

Several witnesses - including the specialist crash investigator, attending police, property owners and Myley's family - are expected to give evidence.

The inquest before Deputy State Coroner Elizabeth Ryan is expected to run for four days.

It will also examine whether relevant quad bike riding regulations were complied with; whether the quad bike was fit to be ridden; and whether further regulations were needed governing riders and passengers on quad bikes.

It is legal for children to ride quad bikes and there are no regulations for children to wear helmets, the court was told on Tuesday.

Quad bikes are the biggest killer of workers on farms in Australia and there were more than 200 quad bike deaths in Australian between 2001 and 2015. There were 11 deaths, including Myley's, in 2017.