The Tarwin River is the primary river system within South Gippsland Shire and has a catchment area of approximately 1,500 square kilometres (580 sq mi), predominantly rural with small pockets of residential land use.

It flows south from the Strzlecki Ranges and discharges in the eastern reaches of Anderson Inlet, a shallow estuary connected to Bass Strait.

The east branch of the river rises below the locale of Dollar, then heads north through Tarwin East and on to Mirboo before heading south through a valley, breaking out on to cleared farmland north of Dumbalk where the river is up to 12 metres (39 ft) wide with slow flowing water when not in flood; and then continues south.

The west branch of the river rises as a number of tributaries on the southern slopes of the Strzelecki Ranges at the south of Mount Worth State Park.

[5][6] The two branches then form as the Tarwin River, and flows generally south and then west by south, joined by two minor tributaries, through flat open river flats, passing around Tarwin Lower, flooding many local roads and then flowing into an extensive estuary and reaching its river mouth, emptying into Andersons Inlet, which flows into Bass Strait, near the township of Venus Bay.