Leslie Armitage (23), of Tipperary, was the driver of a Land Rover Discovery which failed to take a moderate bend on Sullivan Rd, a gravel road in the Ida Valley, about 13km from Omakau.
Mr Armitage was thrown out and trapped under the vehicle when it rolled into a paddock.
He died at the scene.
It was a dire weekend on the country's roads, with 11 people killed, including two cyclists and a heavily pregnant woman.
The Poolburn accident happened about 1.30am on Saturday and Sgt Kerrisk said alcohol and not wearing a seat belt were factors.
Police said a group of six people - four Irishmen working in this country and two Central Otago women - had been drinking at a nearby hotel and left in two vehicles to travel in convoy to a house on Moa Creek Rd, south of Poolburn.
The women and two Irishmen were in the first vehicle and Mr Armitage and a 21-year-old Irishman followed behind in the Land Rover.
The driver of the first vehicle, a 35-year-old Poolburn woman, failed to negotiate the same bend in the road where the fatal accident happened a short time later, and drove off the road, narrowly missing the water table.
She managed to steer the vehicle back on to the road and continued driving, Sgt Kerrisk said.
Following a little later, Mr Armitage ran into the water table and the Land Rover rolled.
His passenger received minor injuries and was treated at the scene.
The occupants of the first vehicle returned to find them a few minutes after the crash and travelled to a nearby farmhouse to contact emergency services, as there was no cellphone reception at the site.
Sgt Kerrisk said the first vehicle had nothing to do with the fatal accident, but the female driver had been charged with drink-driving and would appear in the Alexandra District Court later this week.
The death of Mr Armitage would be referred to the coroner.
"While it is not my role to usurp his role, it is tragically evident that drink-driving and failing to wear seat belts played a part in this accident, " he said.
Mr Armitage and his passenger had not been wearing seat belts "and it's our understanding, from talking to witnesses, the people in the first vehicle were not wearing seat belts either".
"For many years, police and road safety authorities have been highlighting the issue of drink-driving and failing to wear seat belts, especially on rural roads," Sgt Kerrisk said.
Police would also be talking to the manager of the hotel concerned, he said.
Three of the Irishmen have been working on Ida Valley Station for the past fortnight as farmhands.
One of the men, Ciaran Murphy (22), had been friends with Mr Armitage since the pair met three years ago at Gurteen College in Tipperary, where they recently completed honours degrees in land management.
Mr Armitage had always planned to travel around New Zealand once he completed his studies.
"That was always his goal from the day we met, end of story," Mr Murphy said.
They planned to travel around New Zealand and Australia before Mr Armitage returned home to take over his family farm.
The men had been in this country since the start of November, touring the South Island, and recently went skydiving and bungy jumping in Queenstown.
"Leslie absolutely loved that, we all did ... I'm glad he got to do that," Mr Murphy said.
"He's the very best friend you could hope to have - nobody would say a bad word about him.
"He was easy to get on with, worked hard and enjoyed his time off. He would always do you a favour if he could - nothing was a problem."
Mr Armitage was passionate about farming and loved working in Central Otago.
Mr Murphy, who was travelling in the first vehicle on Saturday morning, declined to comment on the crash, except to say "It's just a nightmare, and you keep thinking you'll wake up."
The Poolburn community had rallied around to support the men grieving for their friend, he said.
Mr Armitage's employer, Callum Paterson, said the young man was "a real top bloke".
"He was a real character, very hard-working, and any spare moment he had he'd be laughing about something."
