The 21-year-old woman was crossing Leakes Road in Tarneit near the intersection of Penrose Promenade around 5.45pm yesterday when she was hit by a white sedan.
The driver did not stop the car after striking the woman, and instead sped away from the scene in an easterly direction towards Derrimut Road.
Another motorist stopped to help the woman and called for an ambulance.
The Tarneit woman was taken to the Royal Melbourne Hospital in a serious condition with a head injury.
A Tarneit man, aged in his 20s, has this morning approached police and is helping with the investigation.
Anyone with dashcam vision or who was in the area at the time is urged to contact police.