A motorcyclist has been seriously injured when he was thrown from his bike while trying to avoid a 4WD in Sydney's south west.

Police are searching for the driver of the 4WD, who drove away from the scene of the Bankstown crash on Saturday morning.

It's believed the 4WD swerved to avoid another vehicle before approaching the motorcycle.

The 23-year-old rider was thrown after the motorcycle struck the kerb of Marion Street, mounted the footpath and hit a brick and concrete fence.

Paramedics took the 23-year-old to Liverpool Hospital for surgery after he suffered serious lacerations to his leg.

© AAP 2014