The four teenagers were driving a Ford Explorer south on wing street, in windsor , when the car crashed and ended up on its roof in a roadside drain at 6am (AEDT) on saturday.
There were no obvious hazards where the accident happened and the road is wide-open and straight, windsor police duty officer Inspector John Woodsworth said.
Investigators will look at the possibility that the car swerved to avoid an animal before it crashed, among a host of other possibilities.
"It will be some time before we know, particularly given the damage to the car, which was substantial," Insp Wadsworth told AAP.
"What I can say though is that from the gouge marks on the road, the car rolled over and over."
An 18-year-old man died at the scene while another 18-year-old male was flung from the car and went into cardiac arrest. He died on the way to Westmead Hospital.
Two teenage boys, aged 16 and 14, suffered injuries which were not life-threatening. All four were friends and are understood to have lived locally.
The Ambulance Service of NSW initially said the 16-year-old was driving the car when the accident happened.
However police said it had not been established who was at the wheel.
The two surviving teenagers were due to be interviewed by detectives
my brother was the 18 year old male killed at the scene this