It is approximately 1,300 ha (3,200 acres) in extent, and up to 5 m (16 ft) deep. It is fed by the Deep and Frankland Rivers, and communicates with the Walpole Inlet via a natural channel approximately 1 km (0.62 mi) long and 2 m (6 ft 7 in) deep.
The esutary is wave dominated and is mostly not modified but the catchment has been substantially cleared. The inlets have a naturally low turbidity but have a high sediment trapping efficiency. The inlets have a total area of 15.8 km2 (6.1 sq mi), most of the area being found in the central basin. The inlet has a mean depth of 2 m (6 ft 7 in).
The Walpole/Nornalup inlet system is the only permanently open estuarine system in the South West, giving it great biological diversity.
Capt. Thomas Bannister and his party visited the inlet in 1831, although sealers had been based in the area since before 1826.
Both estuaries, with the tidal parts of the tributary rivers, are now the subject of a proposed marine park.
- Thomson-Dans, Carolyn (December 2006). "Marine Park Proposed for the Deep South". Landscope. WA Dept. of Environment & Conservation.
- "Walpole-Nornalup Coastal Attractions". 2010. Retrieved 19 April 2011.
- "Estuary Assessment Framework for Non-pristine estuaries - Estuary 646 - Walpole/Nornalup Inlet". 2004. Retrieved 19 April 2011.