Police in New South Wales are investigating hit-and-run incidents yesterday that have left two cyclists in intensive care in hospital.

Brendon Braid was riding south along the Old Princes Highway at Helensburgh, south of Sydney, when he was hit at about 6:15am (AEDT).

A group of cyclists found the 58-year-old badly hurt by the side of the road a short time later, but no sign of the car that had hit him.

Paramedics treated the man for several suspected fractures before taking him to St George Hospital.

Police say the man remains in intensive care in a stable condition.

He has a broken ankle, broken leg, cracked lower vertebrae, internal bleeding and a badly grazed and bruised face.

Investigators are searching in particular for the driver of white Ford Laser hatchback with Victorian number plates and green P plates.

They are calling for further witnesses or anyone with information about the driver's identity to come forward.

Police are separately seeking the driver of a truck that his a woman as she rode in Sydney's west at Kemps Creek.

The 26-year-old was thrown from her bike when it was struck by the truck on Mamre Road just before 10:30pm.

The truck did not stop.

Paramedics took the woman to Liverpool Hospital, where she remains in a stable condition in the intensive care unit.