A cyclist was struck by a vehicle and flung from his bike in a hit-and-run crash that left him lying by the side of the road south of Sydney with a broken back.

Brendan Braid, 58, was found by a number of cyclists several minutes after the crash on the Old Princes Highway at Helensburgh early on Sunday.

The remnants of Brendan Braid's bicycle after he was struck by a vehicle and flung from his bike on the Old Princes Highway at Helensburgh on Sunday. Credit:Southern Cross Cycle Club, Facebook

He was curled up beside his mangled bike with fragments of a car mirror scattered around him, and is believed to have been knocked unconscious.

Mr Braid, a member of the Southern Cross Cycle Club, was taken to St George Hospital.