State Highway 1 is now open in both directions following a serious crash involving a caravan near Wellsford.
Motorists are advised to continue to expect delays as traffic congestion eases.
The incident happened shortly after 1pm between Wellsford and Warkworth, near Wayby Valley Rd.
A woman was seriously injured and is being airlifted to hospital, police said.
Three other people have been taken by ambulance to North Shore Hospital, one in a moderate condition and two with minor injuries.
A St John spokeswoman said two ambulances and a helicopter had been sent to the scene.
The NZ Transport Agency said earlier that SH1 was closed between Wayby Valley Rd and Kaipara Flats Rd.
"Detour routes are being set up and traffic at the crash scene is being directed by emergency services. Please consider using SH16 and expect delays," it wrote then.
Northbound commuters reported then that traffic was at a standstill through the Dome Valley, and the queue was quickly growing.
The 19-year-old disappeared without her wallet or mobile phone.