Police were called to the crash about 2pm.
Senior Constable Mike Colligan said the 69-year-old woman, who was believed to be from Ettrick, was pronounced dead at the scene.
Her name has not been released as next of kin have not been notified.
The truck driver was uninjured.
Emma Curtis, from Palmerston North, who is working with a Department of Conservation group for the summer, was at Butchers Dam with Doc staff when she and others heard a loud bang.
Doc staff raced to the woman and began doing CPR, she said.
Police praised the actions of the Doc staff, who continued trying to help the woman until the ambulance arrived.
"They did an outstanding job in trying circumstances," Snr Const Colligan said.
The workers, who were upset by the crash, did not want to talk to the media.
The road was closed for more than an hour while crash investigators carried out a scene examination.
A large grader was brought in to cut a detour around the crash site to help move traffic on the busy highway.
It would be some time before the cause of the crash was known, Snr Const Colligan said.
Speed and alcohol were not believed to be factors at this stage.
A teenaged motorcyclist who crashed into a bridge at Taiko, 19km northwest of Timaru, last Thursday has died in Timaru Hospital, NZPA reports.
He was Kevin Barry Van Turnhout (16), of Albury, near Fairlie.
Police have named the motorcyclist killed in a collision with a bus in Auckland on Monday.
He was Reinhardt Marais (18), from Glen Eden.
