Senior Sergeant Graham Shields, of Thames police, said the Waikato serious crash unit was investigating. Officers were at the crash scene for about three hours doing a scene examination.
Mr Shields said it didn't appear anyone witnessed the accident.
Members of the Ngatea Volunteer Fire Brigade were the first on the scene and found the male driver conscious in his wrecked vehicle, brigade member Adam Shelley said.
Debris was strewn around the area, he said, but neither the injured man nor the dead woman needed to be cut from the cars.
Mr Shields said the injured man was taken to Thames Hospital with minor injuries and was expected to be discharged last night.
Police in South Auckland are looking for witnesses to a crash at the intersection of Ranfurly and Mill Rds, Alfriston, that killed a female passenger and put two others in hospital. Two vehicles collided about 11.45pm.
The passenger died almost instantly but the driver was unhurt, said Senior Sergeant Emiel Logan.
"He was the only person we could speak to last night.
"He was distraught and was trying to help us but he's absolutely devastated about what happened."
Two others from the other vehicle were in Middlemore Hospital with moderate injuries and had not been spoken to by police.
"We're obviously looking for witnesses," Mr Logan said. "Mill Rd is quite a busy stretch of road so we just need some help piecing together what happened."
The woman's name would not be made public until her family had been advised, the senior sergeant said.
The third weekend crash victim was a 21-year-old North Otago man who died after his vehicle crashed into a bridge about 4.20am yesterday.
The accident happened on State Highway 83, on the Otematata-Kurow Rd, 95km northwest of Oamaru.
Senior Sergeant Anton Tenhove said there was no indication that poor weather was a factor in the crash.
- additional reporting: NZPA
