Two girls have been arrested and two people are still on the run after a high-speed chase involving a luxury car on a busy highway north of Melbourne.

The driver, believed to be a 15-year-old boy, was seen driving up to 120km/h through Victorian towns, narrowly missing pedestrians and almost colliding with a semi-trailer.

He is one of two people known to police still on the run on Friday night.

Police first spotted the car, which looks to be a BMW, driving erratically in Thomastown about 2pm before it headed to Whittlesea.