A subsequent WorkSafe investigation found the condition of the tow ball and tow coupling was a "significant concern". There was "very extensive wearing" on both, which meant a small bump in the road, or change of incline, could allow the trailer to easily decouple.
The trailer also had uneven tyre pressure.
When the crash happened the truck driver Johnnie Johnston was in breach of work-hour regulations, having driven for about six and a half hours without a break.
He told investigators he did not realise the trailer had detached until he heard a thud and saw wreckage in his rear-view mirror.
WorkSafe said Johnston's Direct Logistics failed to undertake regular and effective inspections of its vehicles, and failed to identify the deterioration of the tow ball and coupling.
WorkSafe area manager, Danielle Henry said, "The company had a duty to ensure the health and safety of other people was not adversely affected by its work. Johnston's should have been doing regular inspections of all its vehicles including the trailers couplings and tow balls to ensure they were safe and roadworthy."
The company should also have ensured its vehicles had current warrants or certificates of fitness. Johnston's should have identified and logged the maximum weight every towing vehicle and trailer could manage, to ensure that towing componentry was rated safe for use.
"This tragedy should serve as a warning to other businesses to keep a much closer eye on basic maintenance. A young girl's life has been lost through no fault of her own, and her whānau is forever impacted," Henry said.
In court, Johnston's director said he had not stopped thinking of the crash and could not begin to understand how the family felt. The driver had been badly affected by the accident and had not driven since.
"We're a small family business, we get by day to day and we know any penalty the company receives or reparation the company is ordered to pay will not be adequate replacement for the life of Storie."
