How it worked in 1888... then in 1935

How it worked in 1888... then in 1935

Nepean System diagram

The Nepean System schematic diagram.


  • Rainfall on the Illawarra Plateau south of Sydney flows into the Cordeaux, Cataract, Avon and Nepean rivers.
  • A weir at Pheasants Nest, just below the junction of the Cordeaux and Nepean rivers, diverts some of the water into the 7km Nepean Tunnel which travels under the town of Wilton and connects with the Cataract River.
  • A second weir on the Cataract River at Broughtons Pass diverts water into the 3km Cataract Tunnel.
  • Water emerges from the Cataract Tunnel into a canal at Brooks Point, near Appin and then continues to flow by gravity to Prospect Reservoir.
  • The entire 64-kilometre length of open canals (44km), tunnels (19km) and aqueducts (1km) is known as the Upper Canal.
  • From Prospect Reservoir, water flows by gravity along the 7.7km Lower Canal to a basin at Guildford (known as Pipe Head), from where it is piped to a service reservoir at Potts Hill, near Auburn.
  • From Potts Hills Reservoir water is piped to the Crown Street Reservoir, and a larger service reservoir at Petersham.
  • From Crown Street and Petersham reservoirs, Upper Nepean Scheme water is supplied to the city and suburbs previously supplied with water from the Botany Swamps Scheme.

...and Stage 2 (by 1935)

  • Construction of the four Upper Nepean dams between 1902 and 1935 essentially did not alter how the Upper Nepean Scheme worked - it simply increased the amount of water the scheme could reliably supply, by storing water in upstream dams rather than relying solely on river flows.
  • Water from Avon, Cordeaux and Nepean dams flowed to Pheasants Nest Weir, where it was diverted via the Nepean Tunnel to Broughtons Pass Weir. Water from Cataract Dam flowed directly to Broughtons Pass Weir.
  • From Broughtons Pass water flowed through the Cataract Tunnel and Upper Canal to Prospect Reservoir - as it did before construction of the Upper Nepean dams.
  • The Lower Canal was replaced with a tunnel and was decommissioned in the 1990s. The Upper Canal continues to flow by gravity, as it has done for 125 years.


Sunday 03 July
2,614,900 ML
2,618,706 ML
8,668 ML
1,239 ML
89,916 ML
Sunday 03 July