Emergency services rushed to the scene, on Old West Coast Rd, West Melton about 11.15pm.
A witness said the road, a state highway running through the Selwyn District, was wet at the time emergency services arrived - it had just begun raining.
The cars ended up in crumpled heaps on either side of the highway.
Rural area commander Pete Cooper confirmed there had been one person in each car.
A 60-year-old woman was killed, while the other driver - a teenage boy - was critically injured.
Cooper said officers have been unable to chat with the teen, who is in the ICU at Christchurch Hospital.
The Serious Crash Unit completed a scene examination, Cooper said, and the focus was now to determine why one car was on the wrong side of the road.
“The road is reasonably straight so we wouldn’t call it a high-risk road,” he told The New Zealand Herald.
“Every road is difficult to navigate if you’re concentrating on something else, at this stage we can’t rule out the road as a factor but it’s not a difficult stretch of road.”
Investigations will continue at the scene today. The road has since reopened to the public.
Reflecting on the last year, Cooper pointed out this was the 14th road fatality he’d experienced on his district’s rural roads since January.
He called the toll “horrendous”.
“The amount of trauma it’s caused families in rural Canterbury, they’re suffering,” Cooper said.
“Crashes like this one are all preventable. It’s an annual message, but drivers need to take the act of driving seriously.”
