One of the girls, aged 10, was so badly injured she went into cardiac arrest and had to be revived by paramedics at the scene.

The driver, a 31-year-old Marshall Mount woman, and two other girls – both 11 – all escaped serious injury.

Police have been told the woman lost control of the Polaris Ranger buggy on Calderwood Road about 10.20am.

The vehicle then veered off the road and crashed into a wooden fence.

NSW Ambulance Illawarra district Inspector Terry Morrow said it was believed the most seriously-injured girl, from the Sydney suburb of Birrong, was thrown from the buggy.

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Inspector Morrow said the girl suffered serious head and chest injuries, and was in cardiac arrest when he and the specialist crew from a rescue helicopter arrived.

“We immediately maintained an airway with an intubation tube and identified that the chest injury was probably the most significant of her injuries,” he said.

“Each time we started ventilating her, she started to get worse.

“We identified she had bilateral tension pneumothoraxes; what that means is that both lungs have collapsed, both lungs are bleeding and both lungs are capturing air within the cavity of the chest wall, which then is basically a death sentence within a short period of time.”

The girl again went into cardiac arrest and was resuscitated.

She was given adrenaline to restart her heart and received a blood transfusion at the scene, before being flown to the Sydney Children's Hospital at Randwick in a serious condition.

Inspector Morrow said the helicopter landed in a paddock close to the accident scene, helping to ensure the girl was treated quickly.

“Everyone worked as a fairly good, well-oiled team in trying to keep this girl alive and it was remarkable that we did so well under the circumstances,” he said. 

One of the 11-year-old girls was taken to Wollongong Hospital with a shoulder injury; the other was treated at the scene for minor injuries. The driver was also hospitalised with minor injuries and for mandatory testing.

It is understood all four occupants were sitting on the bench seat of the buggy.

The vehicle has two seatbelts, but it is believed none of them were wearing one at the time.

Wollongong police Inspector Mark Walker said that would form part of the investigation and the vehicle would be mechanically examined.

The Mercury understands one girl was filming the buggy ride on Snapchat when the crash occurred. 

Insp Walker said the vision was being looked at by police.