Northern Fire Communications shift manager Steve Smith said the fire service had sent three vehicles to the crash and firefighters freed one person who had been trapped in one of the cars.
One person was flown by Westpac Rescue Helicopter to hospital with critical injuries and at least two others, including a child, were injured.
A spokesperson for St Johns told the Herald ambulances had taken one person, who was critically hurt, to Auckland Hospital and two people with serious injuries to Middlemore Hospital.
An Auckland DHB spokewoman confirmed the patient, a women, was in intensive care at Auckland Hospital with critical injuries.
Middlemore Hospital's spokeswoman, Lauren Young, said a 29-year-old women and 6-year-old boy had been admitted following the crash. Both patients were in a stable condition, she said.
A police spokesman said traffic will be disrupted for a "considerable time" while the road is cleared and the serious crash unit investigates.
Traffic is being diverted through Pokeno. NZTA spokeswoman Sarah Azam said the traffic was "flowing well" through the diversion and authorities at the scene estimated the crash would be cleared and the road reopened in about 1.5 hours.