A young boy is fighting for his life in Starship children's hospital in Auckland after a motocross accident this afternoon.

The crash is believed to have happened at the Pukekohe Motorcycle Club's Kawasaki Sand Track on Kimikimi Rd in Mercer.

The Auckland Westpac Rescue Helicopter was alerted to the job at 3.45pm, a spokesman said.

Medics had to carry out a Rapid Sequence Intubation - inserting a tube down the boy's throat - to help him breathe, the spokesman said.

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The boy was flown to Starship Hospital in a critical condition.

Pukekohe Motorcycle Club confirmed an incident had occurred but declined to comment. The club had posted to social media yesterday photos of its sand track being prepared for the final round of the Winter Series today.

Private ambulance operator Transitcare New Zealand had posted to social media saying it would be attending the event.