Motorists on one of the country's most scenic highways desperately called police to warn about a vehicle's erratic driving before a fatal crash on Monday.
Monday evening's horror smash on SH8 near Lake Pukaki killed two people, including local Twizel identity Nigel Freeman.
Police received three calls over a 15km stretch of highway from concerned motorists about one of the vehicles involved in the deadly smash.
"It was just the manner of driving to the extent that it caused three other motorists on the road to be concerned with the ability of the driver."
Mid-South Canterbury area commander Inspector Dave Gaskin said police tried to head off the vehicle before anything happened.
This included getting an off-duty police officer in Twizel to intercept the vehicle.
"But unfortunately while he was speaking to the driver he took off and crashed," said Gaskin. The driver survived with moderate injuries.
He said despite their best efforts, police were unable to prevent the tragedy.
"There was nothing more anyone could do. Everything was done to try to prevent it but unfortunately we were not dealing with a rational human being.
"Sometimes the best efforts we can do is just not enough."
He described the Mackenzie Country crash as one of the most serious he had seen for a number of years.
The crash, which closed the highway between Hayman and Tekapo Canal Rd, killed both men travelling in one car and left another in hospital with moderate injuries.
He said it had rocked the close-knit Twizel community robbing it of a man who was well regarded.
Police would like to hear from witnesses to the crash or from motorists who noticed a silver 2017 Toyota Highlander travelling north on SH8 before the crash.
Gaskin refused to say what had caused the crash but urged people not to drink before getting behind the wheel and to wear seatbelts.