The death comes amid a horror eight days on New Zealand roads, in which 22 people have died in 15 crashes.
Polcie say Tony Kelvin Wood (18) was a passenger in the car. It was heading south on State Highway 1 when it braked heavily behind slow traffic, then crossed the centre line into the path of the truck at the north end of the Waitaki River bridge just after midday.
The driver (19) is still in critical condition in Dunedin Hospital, police said.
A man in the horse truck, which belonged to A and F Clark Harness Racing, received minor scratches to his legs and was later treated at Oamaru Hospital. His wife was unhurt.
The two horses in the truck received minor injuries and were treated by a veterinarian.
Mosgiel trainer Alan Clark's horse The Fiery Ginga, one of the country's top 3-year-old trotters, was scratched from a race meeting at Addington last night after being involved in a crash, a spokesman for the New Zealand Metropolitan Trotting Club confirmed.
Senior Constable Clint Vallender, of Waimate, said police wanted to speak to a female motorist driving a white or grey car who might have witnessed the crash.
The Waimate man's death was the 22nd on New Zealand roads since the start of the Easter break, in eight days of carnage the police have described as "horrendous".
Twelve people died as a result of road crashes during the official Easter break. There were three crashes on Tuesday, a double-fatal and a triple-fatal crash on Wednesday and a pedestrian death on Thursday.
Inspector John McClelland, from police national headquarters, said what was so upsetting was that every crash was preventable.
"It's just such a waste of life."
Speed and alcohol were suspected factors in five of the crashes.
"The disappointing thing for many of them is booze and speed, and really police can do only so much, you can legislate only so much," he said.
"People driving vehicles just really have to take responsibility when they're on the road and be aware of what's happening around them and be prepared for the unexpected."
Driver fatigue could also have been to blame for a crash near Mossburn on Wednesday, in which two English tourists were killed, Insp McClelland said.
"Fatigue or tiredness comes on gradually. If you find you're wandering or your eyes are droopy then you've really got to stop.
"You're better off being late than being dead on arrival."
The British couple were the only road deaths in the Southern police district, which covers the area south of the Waitaki River, since the start of Easter.
However, three people were seriously injured when six people, including four children, were thrown from a van that left the road near Garston on Easter Monday.
In the week before Easter, a 75-year-old pedestrian died after he was struck by a trailer at Moeraki and an 87-year-old man died when he drove into the path of a logging truck near Maheno, south of Oamaru.
