The crash occurred when a vehicle collided with the minivan and then veered into another vehicle as the minivan tumbled down the bank. Police closed SH16 between Muriwai and Kiwitahi roads after the crash.
Westpac Rescue Helicopter flight paramedic Marcel Driessen said they flew a man aged in his 20s to hospital.
"They said he should go first and it was a good call. He was conscious but he had abdominal, pelvic, leg and arm injuries. He was in a lot of pain."
He understood another injured person - possibly a teenage boy - had been thrown from the minivan during the crash.
"He was not very well."
Fire Communications shift manager Colin Underdown said volunteer firefighters from Kumeu and Helensville were also sent to the scene and helped free a person trapped in one of the vehicles.
Caltex Waimauku staff member Sam Miller said shaken customers had told him of the aftermath of the crash.
"It's horrible. We've been having so many out here."