It has its origin from Waratah Rivulet, near Darkes Forest, and flows generally north for approximately 36 kilometres (22 mi), joined by three minor tributaries, before reaching its confluence with the Georges River, between Como and Illawong.

The 71,790-megalitre (2,535×10^6 cu ft) impoundment is Lake Woronora, formed to augment water supply for southern Sydney and the northern Illawarra region.

"The Needles" is considered the "head of navigation" where there is relatively deep water with pointed vertical rocks, mostly just below the surface, hence the name.

Until 2008, there was a vehicle causeway there at an area called the Pass of Sabugal, so named by Major Mitchell who surveyed the route in 1843 for what would become part of Old Illawarra Road.

Just above the Heathcote Rd bridge was "Woronora Wier" which was demolished pre-1994 and the roadside parking area removed for safety reasons.