Coroner Richard McElrea has found that Rutger Telford Hale, 22, died after being struck in the head by a small solid object that fell from another vehicle.
The object has never been found or identified, despite an extensive search and scientific analysis of residual material in the vehicle.
In his findings Coroner McElrea said Mr Hale was driving his Subaru Legacy station wagon on State Highway 6 towards Albert Town, early on October 24, 2013.
Mr Hale was travelling with his Alaskan partner Danielle Oylear, who was the front seat passenger.
Coroner McElrea said as Mr Hale's car travelled past the Te Awa Road intersection onto the Maungawera Hill, an oncoming vehicle came around the corner.
It was a dark and rainy night and as the vehicles came level with each other an "unidentified object" passed through the windscreen and out of the rear window of the Subaru, striking Mr Hale on the right side of his face and head.
As the car reached the top of the hill Ms Oylear saw her partner was "slumped" in the driver's seat and the vehicle was not slowing down, so she attempted to take the wheel, and lift Mr Hale's foot off the accelerator, Coroner McElrea said.
However, the car left the road ending up in a ditch, and when emergency services arrived Mr Hale was dead.
The driver of the other vehicle at the time of the accident has never been identified by police.
In his findings Coroner McElrea noted that Ms Oylear said something "didn't seem right" about the other vehicle on the road at the time of the accident.
She said as the oncoming vehicle drew level with the Subaru she saw a " an object flying towards the Subaru".
"It was about the size of a tissue box or brick and seemed to me to be beige in colour.
"My instant thought was somebody threw something but I would have seen an arm or a window down or seen a person.
"I put my arm up in an instinctive reaction and heard a loud noise, which I would describe as a 'crash-thump' and when I opened my eyes I could see the windscreen was badly damaged."
In his findings, Coroner McElrea said Mr Hale's "most unusual" wound was semi circular and about 80 millimetres in diameter.
He said the object was possibly made of stainless steel, was man-made, had a "sandpaper rough" surface and was coated in soil.
It was unlikely a stone or rock had been thrown up from the side of the road, or that the wound had been caused by a gunshot, Coroner McElrea said.
"I find to a high degree of probability that the source of the object was the oncoming utility vehicle and it is likely to have slid off the deck of that vehicle.
"The object did not disintegrate and despite an extensive scene search, was not located.
"It is unlikely that the object will be full identified without significant new evidence being made available to police."
Coroner McElrea made no recommendations.
By Nikki Papatsoumas of NZME. News Service
