The coroner has found the road knowledge questionnaire tourists need to answer before hiring a car is "wholly inadequate".
The findings are in a report into the 2017 deaths of Southland couple Jesse and Samantha Shortland.
Their car was hit when the one being driven by German tourist Marina Liebl crossed the centre line near Lumsden.
Liebl also died.
While both the Shortlands died, their two children - one aged two years, and the other three months - escaped with minor injuries.
Coroner David Robinson fell short of making formal recommendations but encouraged rental car companies to introduce a better assessment of whether overseas drivers were ready to drive on New Zealand's roads.
He said fatigue was a factor in the crash for both drivers, as was their speed.
Five seconds before the crash, Liebl was driving at 134km/hr, slowing to 73km/hr at impact.
The Shortlands vehicle was moving at 119km/hr when they collided.
Robinson said cannabis was found in the system of Jesse Shortland, and while he could not determine his level of intoxication, he could not discount the possibility this may have slowed his reaction time.
Judy Richards, whose son Rhys Middleton was killed by a tourist driver in 2016, welcomed the call for rental car companies to improve their driver assessments.
She said rental car companies needed to lift their game and should require tourists to first sit a practical test.
She would also like to see overseas drivers serve their sentence in this country, rather than being sent home.