Witnesses saw a large truck and trailer weaving across the road before crashing into an oncoming car south of Oamaru, leaving both drivers dead.
The heavy vehicle combination was heading north on State Highway1 when it crashed in Alma, about 5km south of Oamaru, about 9am yesterday.
The highway was closed for most of the day as the serious crash unit and the commercial vehicle safety team investigated.
A witness, who was driving south directly behind the car involved, said the truck veered left and hit a fence before swerving right.
The driver of the car tried to pull over to the left as the truck approached, but was hit in the side.
The witness spoke to others at the scene and heard their statements to police.
One told police the truck driver had been swerving for "about 10 minutes" before the crash and another said they had tried to call the truck driver’s company to warn them something appeared to be wrong with the driver.
The witness pulled over and ran to the aid of the car driver, but nothing could be done.
The truck was carrying coal for Hilton Haulage.
Chief executive Dylan Fitzgerald said he was unable to comment on possible causes of the accident, but would support police in their investigation. An internal investigation would also be conducted.
"Our deepest sympathies go to both families during this time."
There was a significant emergency services response to the crash.
At one point there were four Fire and Emergency New Zealand vehicles, two ambulances and two rescue helicopters at the scene, working alongside police.
It was the second fatal death on Waitaki roads in less than a week and the fourth and fifth fatalities on South Island roads in less than 24 hours.
The New Zealand Herald reported a baby survived a head-on crash at Leithfield, in North Canterbury, in which both drivers were killed on Sunday afternoon.
A person died after a single-vehicle crash on Ōtira Highway, Jacksons, in Westland, about 3.40pm on Sunday.
Stacy Henderson, 43, died in a two-car crash on SH8, between Twizel and Omarama, on Thursday morning.
Police also called on road users yesterday to follow safety advice as thousands of drivers and motorcyclists head south to Invercargill for the Burt Munro Challenge, which runs from February 7-11.
Southland area commander Inspector Mike Bowman said the aim was for those who travelled to the event to return home safely.
"Motorcyclists are vulnerable road users given the much lower levels of protection they have in the event of a crash compared to the occupant of a car."
