A man critically injured north of Kerikeri on Monday afternoon was not wearing a helmet when he fell off a motorbike and hit his head on rocks, police say.

Northland crash analyst Senior Constable Jeff Cramp said the accident was a tragic reminder to always wear a helmet while riding, ''no matter what you're doing''.

The incident occurred about 3.30pm on Te Tii Rd, a few hundred metres from Whitiora Marae, when a group of people were joyriding on a 450cc road bike.

A 22-year-old man was rounding a moderate right-hand bend when he dropped the bike and was thrown into a roadside drain where he struck his head on rocks.


That section of Te Tii Rd is sealed but it is believed the rider hit a patch of gravel on a corner.

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Medics spent more than an hour and a half working on him before he was stable enough to be flown to Auckland City Hospital by the Northland Rescue Helicopter.

As well as critical head injuries the rider had also suffered a suspected broken femur. As of Tuesday morning he remained in the hospital's neurological unit.

Cramp said the crash was a safety reminder for all riders.

''The bottom line is, always wear a helmet on a bike, no matter what you're doing.''

Speed was not a factor in the accident but inexperience and lack of appropriate safety gear were. Alcohol consumption was also suspected.

Police had yet to recover the motorcycle.