About 9.10pm, a white sedan and a green sedan both turned left from President Street into Orrong Road.
It is believed the green sedan pulled out into lane two, directly into the path of a white Hyundai Excel hatchback also travelling in lane two in the northwest bound lanes of Orrong Road.
The driver of the white Excel took evasive action, resulting in the Excel mounting the kerb and colliding with a power pole.
The green sedan did not stop and continued along Orrong Road.
The female driver of the Excel and a passenger, both 17, received serious injuries and were taken to Royal Perth Hospital for treatment.
Police are trying to identify the driver of the green and white cars that turned into Orrong Road before the crash.
Anyone with information, or who saw either vehicle prior to the crash, should call Crime Stoppers on 1800 333 000.