Three people have been "burnt alive" and a fourth man is battling life-threatening injuries after a car flipped and burst into flames near Sydney's Darling Harbour in the early hours of Saturday morning.

Police said emergency services were called to the corner of Harbour and Pier streets after a Nissan GT-R high performance car rolled and caught fire shortly before 3am.

"Speed - and considerable speed at that - along with a loss of control are two contributing factors [in the crash]," Superintendent Paul Pisanos, from Sydney City Local Area Command, said on Saturday.

Superintendent Pisanos said police were "extremely traumatised" after attempting to save two men and a woman who caught alight inside the car and were "effectively ... burnt alive".