The mystery object that killed Rutger Hale while he was driving on State Highway 6 near Lake Hawea last year almost certainly came from an oncoming vehicle.
An inquest into the 22-year-old's death, held in Queenstown yesterday, also heard evidence of an unidentified vehicle stopping and returning to the scene of the fatality.
It has raised more questions around what Coroner Richard McElrea described as a "classic freak accident".
Mr Hale died after being struck in the head by an object which crashed through the windscreen of his Subaru station wagon and exited through the rear windscreen early on October 24, 2013.
The object has never been found or identified, despite an extensive search and scientific analysis of residual material in the vehicle.
Mr Hale's Alaskan partner, Danielle Oylear, who was a passenger in the car, told the inquest she recalled seeing a white 4WD truck or utility vehicle coming towards them.
As the vehicle passed, she noticed a beige-coloured object about the size of a tissue box or brick "rocketing towards us" from somewhere between the driver's window and the back of the vehicle.
Serious crash unit investigator Senior Constable Alastair Crosland told the inquest that several scenarios had been considered and rejected in attempts to identify the object.
However, "the only scenario that fits is an object coming off an approaching vehicle".
Tyre marks on the road were consistent with another vehicle slowing and doing a u-turn at the scene before continuing on its way.
Institute of Environmental Science and Research (ESR) senior forensic scientist Angus Newton said scientific analysis of an orange-coloured particulate material found inside the car indicated the object was stainless steel and contaminated with common South Island soil.
A separate ESR report said the object had a likely length of more than 80mm.
Defence Technology Agency investigator Ryan Brookes did not believe the item was made of steel as it did not penetrate the windscreen at its first impact point.
The object could have been contaminated with stainless steel before or after the accident, he said.
Mr McElrea reserved his findings.