The toddler and his mother were "extremely lucky" to be alive, but the boy required ongoing treatment for his injuries.
The truck driver, Ioane Etuale of Auckland, was charged with two counts of careless driving causing death and one of careless driving causing injury, but was found not guilty in the Kaikohe District Court last year.
In findings released this week, however, Mr Shortland ruled the truck had lost control as a direct result of driver error.
"The facts further infer that Mr Etuale was inattentive for a moment of time and once the truck's brakes locked up, sending the truck and trailer into a slide, it was impossible to retract and rectify ... The Spanish tourists were powerless to change their course ... They could not have avoided this unfolding situation as their options to take evasive action were zero," Mr Shortland said.
A 30m section of the southbound lane where Mr Etuale lost control was in poor condition with a pattern of ruts and corrugations known as "crocodile cracking". It was known for being difficult to handle, especially for an unladen truck like the one Mr Etuale was driving. "Uneven surface" signs were erected the next day and the highway was later repaired.
Expert witnesses told the inquest the rough stretch of road could catch drivers unaware, but with common sense and care it should not have posed any great difficulties.
Mr Etuale did not attend the inquest and could not be found by police, but had earlier told the Kaikohe District Court that the truck's steering wheel had suddenly started shaking and he had heard something snap before losing control.
However, Mr Shortland could find nothing to back up Mr Etuale's claims.
While the trailer was not up to certificate of fitness standard, no mechanical faults with the truck's steering or brakes had been found which could have contributed to the crash, despite detailed investigations.
