Howard Willis Case (33) drove off Old West Coast Rd and into a concrete culvert, about 8km from Darfield, on May 3 in 2019. He died at the scene.
Coroner M Duggal said Mr Case was not wearing his seatbelt correctly, had taken drugs - including methamphetamine, ecstasy, LSD and cannabis - and was speeding.
The conditions were foggy at the time, limiting visibility.
Mr Case had driven to Christchurch with two friends, arriving about 7pm. He met up with them again at 10pm and they started to drive back to Greymouth.
Both friends told police they were wearing their seatbelts on the way back, but Mr Case only had his around the waist and behind his back.
They recalled that he was driving fast and both had asked him to slow down.
Mr Case crossed the centre-line, turning the steering wheel to get back in lane, but did so too fast and drove off the left-hand side of the road.
One of the friends recalled hearing loud bangs and then blacking out. The two passengers had to climb over Mr Case, getting out of the car through the driver's window.
One of the women noticed that Mr Case was unresponsive. Blood was streaming down his face, he had a significant head injury and they thought he was dead.
They called emergency services, which took both women to Christchurch Hospital by ambulance.
Mr Case was pronounced dead at the scene.
Following a Police Serious Crash Investigation, the report noted the incident happened at a slight bend in the road, which could be negotiated at the posted speed limit of 100kph.
The report said Mr Case did not recognise the slight bend in the road, probably due to the poor visibility, and had crossed the centre-line. As he was travelling too fast to safely correct, the vehicle rotated to the left towards the mound on the southern side of the road, rolled and became airborne.
Mr Case was partially ejected from the vehicle as it rolled. If he had worn his seatbelt, he may have survived the crash.
The report also said he had imbibed a number of illegal substances resulting in low sodium levels in his blood, which was likely to be a factor in the crash.
Speed was another contributing factor. The vehicle's speed was calculated as between 144kph and 150kph.
The coroner ruled that Mr Case died of high energy impact injuries to his head, brain and upper cervical spine from the crash.
"The dangers of driving while under the influence of drugs, not wearing a seatbelt correctly and speeding are well known.
"I support and endorse the efforts and campaigns of the New Zealand Police and other authorities to publicise the risks of driving while under the influence of drugs, without a seatbelt and driving above the posted speed limit," Coroner Duggal said.
