The police were investigating how a car left the road and summersaulted onto its roof near Tarago, north of Canberra, on Sunday afternoon.
Three passengers - two women aged 18 and 20 and a 20-year-old man - were killed in the crash but the 21-year-old male driver survived. Police said that no other vehicle was involved.
Multiple vehicles from emergency services, including a helicopter, attended the crash site at the intersection of Bungendore and Collector roads early in the afternoon. The three passengers were trapped in the car but died at the scene.
But the driver was able to get out of the vehicle unassisted. He was taken to Canberra Hospital with minor injuries. Police said he would also undergo mandatory blood tests.
Police said on Monday morning that his condition was unknown.
NSW Police said several vehicles attended the incident. NSW Ambulance, rural fire brigades and a Toll helicopter were also on scene.
The Tarago Bushfire Brigade posted on social media that attending "crashes like this are tragic".
"They not only affect the lives of those involved but also the community members who were first on the scene, our volunteer firefighters, local paramedics, police, fire and rescue and Toll rescue helicopter crews," the post said.
Specialist crash officers from the NSW Police Crash Investigation Unit remained at the scene on Sunday night conducting inquiries. Police will prepare a report for the coroner.
Bungendore Road remained closed between Collector Road and the Kings Highway and north to the Tarago township on Sunday night.
Anyone with information about the crash is urged to contact Crime Stoppers on 1800 333 000.
