The driver of a luxury car who allegedly knocked a motorcylist off his bike and left him for dead remained on the run on Monday morning.

Police are investigating the circumstances surrounding an alleged hit-run in South Yarra about 4.35pm on Sunday.

Investigators were told that the motorcyclist, a man in his 20s, was travelling south along Williams Road. As he approached the intersection at Bruce Street, an Audi pulled out and hit the rider.

The Audi then drove from the scene.

The rider was taken to hospital with serious injuries.