A woman in her 20s has been rushed to hospital after the dirt bike she was riding collided with another.

The incident happened about 6pm at Mangatangi, near the Bombay Hills south of Auckland.

A spokesman for Auckland Westpac Rescue Helicopter said it was a "head on collision of two dirt bikes".

"We assisted a female patient in her 20s who had suffered head and chest injuries.

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"She was transported to Middlemore Hospital in a serious condition."