The collision occurred in Waipu shortly after 1.30pm today.
The route has been the main one for cars travelling to and from Northland due to road works on State Highway 1 over the Brynderwyn Hills.
Police are warning drivers to avoid the area around Cove Rd.
The St John ambulance service confirmed that three patients have been critically injured and four seriously hurt.
It said nine vehicles were sent to the scene to treat and transporting seven patients.
Patients were taken to Starship Children's Hospital in Auckland and Northland Hospital.
Three helicopters were dispatched, five ambulances, two rapid response units and two operations managers.
Whangarei's mayor Vince Cocurullo said the road the crash happened on was not usually dangerous.
Up until now, the detour route had been going smoothly and traffic has travelled well below the speed limit, he said.
Waka Kotahi/NZ Transport Agency said the crash happened near Johnson Point Rd and has closed Cove Rd between McLean Rd and South Rd.
"Delay your journey or allow extra time for [an] alternative detour."
One dead in separate crash
One person has died following another crash in Northland this afternoon.
The two-vehicle crash happened at the intersection of State Highway 1 and Apotu Rd in Kauri and was reported to police at 3.13pm.
Police said one person was critically injured and died at the scene.
"Police extend our condolences to their family and friends at this time."
The road has been closed and diversions were in place through Saleyards Rd and Vinegar Hill Rds.
"The Serious Crash Unit will conduct a scene examination, and motorists are asked to avoid the area."
