A woman has been charged after she allegedly hit a cyclist south of Sydney earlier this month and drove off without helping him.

Police say Brendan Braid was knocked from his bike while riding south along the Old Princes Highway at Helensburgh about 6:15am (AEDT) on Sunday January 5, 2014.

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

Mr Braid was treated by paramedics then taken to St George Hospital with a broken ankle, broken leg, cracked lower vertebrae, internal bleeding and a badly grazed and bruised face.

Police revealed after the crash that they were looking for a white Ford Laser hatchback with Victorian number plates and green P-plates.

Inspector Jim Friday says a 23-year-old woman was charged last night at Sutherland Police Station.

"Wollongong Police made inquiries in relation to information received, conducted an investigation and with that information now at hand arrested that 23-year-old female from Engadine," he said.

The woman has been charged with negligent driving causing grievous bodily harm and failing to stop and assist.

She has been granted bail to face Wollongong Local Court next month.