Police said the child was with his mother, two siblings and a friend, walking along the steep Jump Rock bush trail - about 10 kilometres west of Albion Park - on Sunday afternoon.

The boy slipped and fell about 20 metres, landing on his head at the base of a cliff which fractured his C1 (atlas) vertebra, paramedics said.

He also suffered a fractured arm and bleeding on the brain, but was lucky not to "compromise" his spinal function.

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NSW Ambulance Illawarra Chief Inspector Terry Morrow said the type of neck fracture the boy had could lead to death or quadriplegia, so it was a delicate operation.

"Thank God for the helicopter coming over the top and making access," Inspector Morrow told the Mercury.

"It was a difficult extraction from the scene and treatment of the patient whilst in that environment.

"You're in a creek bed with jagged rocks, you've go to do some serious medical procedures on a young child that you normally don't do in a bushland setting."

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It took paramedics about 25 minutes to walk through "very challenging" terrain to find the patient, while a Toll helicopter from Albion Park winched in a doctor and intensive care paramedic to the creek bed to begin treatment.

Inspector Morrow said crews then had to carry the unconscious boy on foot to a safer area, so the ambulance helicopter could eventually winch him out without hitting fallen trees or other debris.

The critically injured child was then airlifted to the Royal Children's Hospital in Randwick.

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Officers from Lake Illawarra and Southern Region Police Rescue worked with paramedics, Fire and Rescue NSW, and SES and RFS volunteers at the scene.

Mandy Liu was bushwalking with her partner at the time of the accident and witnessed the aftermath.

She didn't have a first-aid kit handy, but was thankful for some "brave" walkers who did and rendered assistance until paramedics arrived.

"The area where accident happened was very steep and there was lots of exposed roots of trees and it was very narrow," she said.

"If you're not an experienced bushwalker you can trip easily."

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Jenae Johnstone from Bushwalk the Gong said the popular walking track is not on the National Parks and Wildlife management plan and has difficult and sometimes dangerous terrain.

"You have to have a degree of navigation involved, and the track in has a lot of slippage," Mrs Johnston said.

"People have put ropes in place to assist to getting across, but ... there's a lot of huge boulders in that area, there's sheer cliff faces."

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