A police spokesman said police, fire and ambulance were on their way to the scene, near Sheepworld - between Christine Place and L Phillips Rd.
State Highway 1 reopened just before 10pm tonight, following the fatal crash that closed the road at 4.41pm.
Emergency services were called to the scene about 6m south of Wellsford near Wayby Valley Rd.
The Westpac Rescue Helicopter has flown two others to Auckland City Hospital, both in a serious condition.
A third person has been taken by ambulance to Wellsford Medical Centre, St John spokesman Robbie Walker said.
Earlier, a fire service spokesman said the driver of one of the vehicles was trapped and the brigade was working to remove them.
He said there was one other person in the same car, but was unsure of the number of people in other vehicles involved in the crash, or how serious any injuries were.
Earlier reports from police said the crash was in the Dome Valley, near Sheep World.
SH1 is now closed between Warkworth and Wellsford, at Kaipara Flats Rd heading north, and Wayby Station Rd, heading south.
Motorists are advised to use SH16 as an alternative route and detours are in place via Woodcocks Rd and SH16.
NZTA suggests motorists allow an extra 40 minutes for their journey.
The section of road is likely to be closed for at least 3 hours, possibly more, according to the agency.
The 19-year-old disappeared without her wallet or mobile phone.