Auckland Rescue Helicopter Trust communications manager Lincoln Davies said a young boy had suffered a head injury.
"He was transported by ambulance to Bay of Islands Hospital alongside our intensive care paramedic who needed to perform a rapid sequence intubation to assist his breathing," Davies said.
Their Westpac 2 helicopter relocated to the hospital, and flew the boy to Starship Hospital in Auckland.
Northland Emergency Services Trust assisted a woman who was also in a critical condition. One other person received serious injuries, and two more had moderate injuries. They were all taken to Bay of Islands Hospital.
Police said the Serious Crash Unit would be investigating.
A Fire and Emergency New Zealand spokesman said two crews attended the accident and extricated one person from a vehicle.
The other car was engulfed in flames upon arrival but was soon extinguished by FENZ crew.
The road had been closed after the crash but had reopened at 12.45am this morning, the NZ Transport Agency said.