Police spokesman Inspector Paul Jeremy said the wreckage was so bad officers struggled to determine which vehicles those who died had been in.
The car crossed the centre line on a 65kmh limit stretch and slammed head-on into the southbound vehicle.
Three in the back seat of the car were reportedly thrown out of the vehicle and died at the scene. The driver also died at the scene.
Wairoa Police Sergeant Tony Bates said police received reports the car had been seen speeding just before the crash and the crash investigation unit was appealing for any witnesses who may have seen either vehicle.
A relative of one of the teenagers killed told the New Zealand Herald she saw a helicopter circling nearby and hoped there hadn't been a crash - then 20 minutes later her daughter called to tell her that her grand-nephew Raimon had been killed.
She said the family had very few details of what happened - just that the car the teenager was in landed in a ditch and that he had died.
"We have no idea what those boys were [doing] out there. We're just trying to work out what happened,'' the woman said.
The family had been plagued by tragedy. Raimon's father died in a car crash before his son was born, then soon after, the boy's grandfather drowned while fishing.
"This boy's father, he was killed in an accident on the Wairoa roads, a terrible accident ... when my grand-nephew was still in the womb. [His mother] had to raise that boy all by herself," she said.
"And then [his grandfather] was drowned in Hawke's Bay when he was out fishing with some friends, not long after. It's really so sad. Now they've lost their grandson ... One woman has lost her son, then her husband, now her grandson."
The woman said her grand-nephew was a "lovely boy".
Police said it appeared many of those involved in the crash were from the Wairoa area.
The families of some of the victims had travelled to the scene of the collision and were distraught.
