An investigation into how a set of tyres flew off a truck and crushed a car on State Highway 2 has proven inconclusive.
Police commercial vehicle investigation unit Sergeant Dennis Hogan said police were unable to establish how the wheels came off the truck-and-trailer unit, which was operated by Container Movers Ltd, as it travelled along SH2 near Bruce Rd on August 13.
He said the lock washer, a locking mechanism that goes between the two nuts on the wheel, did not appear to have been in place.
"They found the two ... nuts but did not find the lock washer," Mr Hogan said.
The truck was serviced the week before the accident and the mechanic was adamant he replaced the lock washer, Mr Hogan said.
Video footage from the workshop that serviced the truck showed no pieces left on the floor or the bed of the truck once the mechanic had finished working on it.
"What happened from there to the time of the crash is anybody's guess," the sergeant said.
Investigators found the nuts in the grease cap but the locking mechanism was not with them.
"If the lock washer was there [to start with] you would expect it to be in the grease cap," Mr Hogan said. "It leaves us in a little bit of a quandary. We have insufficient information to charge either the mechanic or the workshop."
Director of the Hamilton-based Container Movers group, Craig Foster, said he had yet to see the police report but believed the wheels came off due to a mechanical failure.
Investigations were ongoing and it had not yet been established if it was human error or if the washer disintegrated.
"It's certainly not something we wanted to be involved in," Mr Foster said. The company was speaking to third party workshops to look at quality assurance.
A month after the accident the driver of the crushed car, John Robinson, is still feeling the effects.
He now has to walk his epileptic granddaughter 5 kilometres to kindergarten every day but he is just glad to be alive.
Mr Robinson, 57, was driving his daughter's silver Mitsubishi when the two rear wheels came off the truck-and-trailer unit and slammed into the front of his car.
The impact of the tyres, estimated to be travelling at a speed of 90km/h, caused the front of his car to crumble inward, the engine to be pushed upward and the bonnet to bend and restrict his line of sight.
The family are still down one car as insurers battle over who was to blame. The car was not insured.
Rain, hail or shine Mr Robinson has no choice but to push his granddaughter about 5km to kindergarten in her pram.
"It's a bit of a bother being a car down," he said. "But as I say, I'm glad to be alive."
"I'm pretty sore. The shock from the steering wheel travelled up through my arm and buggered my collar bone and chipped my vertebrae."