Jesse and Samantha Shortland, both 28, and 22-year-old Marina Liebl died in the crash on SH6, on the Dipton-Winton Highway, in the Benmore area, on October 8, 2017.
The Shortland’s young children, Skylar, and Heath, survived the crash.
Liebl was travelling on SH6 when she collided with a southbound car driven by Jesse Shortland in about 9.40pm.
Coroner Robinson's report said both drivers were travelling long distances "ill suited to being done in a single day".
Beyond that, it highlighted the fact that while Ms Liebl had filled out a “Safe Drive” form when she rented her vehicle, it would be her first time driving on the left-hand side of the road.
The form acknowledged her familiarity with New Zealand road rules, that she had driven regularly in her home country, that she felt well prepared to drive in New Zealand and that she had driven a similar vehicle in the past.
The coroner said while it was not likely she was travelling on the wrong side of the road for an extended time, her vehicle crossed the centre line.
It was found, had Ms Liebl stayed in her lane, death could have been prevented.
However, factors including speeding, cannabis and fatigue all contributed.
Vehicle data showed Liebl's vehicle was travelling at 134kmh five seconds before impact, before last-second braking reduced the speed to 73kmh.
The Shortlands' vehicle was estimated to be travelling at 119kmh.
While there was no alcohol involved, Mr Shortland's blood tested positive for cannabis.
"Though I cannot determine the extent of impairment (if any), the absence of a braking response, or other evasive manoeuvre is consistent with the known effects of cannabis, including taking longer to respond, and distorted perception."
Given the circumstances of Ms Liebl's involvement in the crash, coroner Robinson reviewed crash data and research publications relating to overseas driver involvement in fatal and injury crashes in New Zealand.
"Ultimately, I cannot reach any conclusions as to whether overseas drivers are overrepresented in crash statistics such as to enable me to consider making recommendations."
However, Mr Robinson described the self-assessment “Safe Drive” form as being "wholly inadequate", with there being no effective evaluation of the person’s ability to drive in New Zealand, nor of their knowledge of the road rules.
"While I cannot make a sufficient link between the “Safe Drive” form process, the apparent lack of knowledge of overseas drivers of New Zealand road rules and the circumstances of this crash in order to make formal recommendations or comments, I would certainly encourage rental car companies to reflect on the research cited and my remarks above and consider introducing some qualitative assessment into the process for determining whether to rent a vehicle to an overseas driver."
