Two cyclists have been knocked off their bikes and left lying by the side of the road with serious injuries in separate hit-and-run crashes on the same day in Sydney.

Both cyclists remain in intensive care in hospital, one with a broken back and the other with head and neck injuries, as police hunt for the drivers involved in the collisions.

Brendan Braid's mangled bicycle after he was hit near Helensburgh. Credit:Southern Cross Cycle Club, Facebook

The latest crash occurred at 8.40pm on Sunday when a 26-year-old woman was hit by a truck at Kemps Creek in Sydney's west.

Police said the woman, who was wearing a helmet, was riding along Mamre Road about 8.40pm when she was struck from behind.