An Auckland rescue helicopter has had a near miss with a drone after taking off to fly to a Northland car crash where multiple patients were waiting.

The near miss happened on Sunday when the Auckland Westpac Rescue Helicopter lifted off from Mechanics Bay at 8pm on the urgent mission.

Pilot James Tayler said the chopper had climbed to 400 metres in altitude and was flying at 230km/h when the crew saw a drone pass down its right hand side about 3-4m away.

"If we had hit the drone, it would have definitely done serious damage to the helicopter, possibly catastrophic if it had hit a rotor blade or other critical component," he said.

"If it had hit the windscreen, it would almost certainly have broken it and injured the crew."

The two-car crash the helicopter was attending occurred on State Highway 1, north of Moerewa, between Hautapu Rd and Nisbet St, about 7.30pm.

Following the crash, a young boy was flown to Starship Hospital with critical head injuries, and emergency services also assisted a woman in a critical condition.

One other person also received serious injuries, while two more had moderate injuries. All were taken to Bay of Islands Hospital for treatment.

When emergency teams first arrived at the crash scene, one person was trapped in a vehicle while one of the cars was also "engulfed" in flames.