Completed in 1935 under the supervision of Ernest Macartney de Burgh, the dam is currently managed by the Sydney Catchment Authority and is listed on the New South Wales State Heritage Register.

In November 1918, a Board of Experts was appointed by the Government to draw up recommendations for the amplification of the Sydney water supply.

It was upon the recommendations of this body, as approved by the Public Works Committee of the NSW Parliament, that not only Cordeaux, but also Avon and Nepean Dams were built.

This avoided transhipment from the Government vehicles bringing materials from large commercial quarries, effectively making the line an extended privately owned siding.

At this time it was realised that scouring at the toe of the foundations of the dam required an alteration to the direction of the flow of floodwaters discharged over the spillway weir.

Provision is made for any leakage of water past the copper seals in the contraction joints to be conducted into the galleries and thence drained away.

It is discharged as required into the river bed below the dam and flows to Pheasant's Nest where it is diverted by a weir to the Nepean Tunnel, 4+1⁄2 miles (7.2 km) long, discharging into the Cataract Tunnel, 2 miles (3.2 km) long, emerging into the Upper Canal, by which it is conveyed to Prospect Reservoir.

The area of the former construction village for the Dam remains undeveloped and heavily overgrown, with remnant road borders and evidence of buildings visible on the ground.

[1] The public area has been maintained by the Water Board and its successors, with picnic grounds, landscaped gardens and a children's playground.

Features of the latter area include pleasant plantings of mixed deciduous and evergreen exotic trees and Australian trees enclosing a long raised shelf, which is the remnant railway platform from the temporary construction railway, with a series of small timber picnic shelters with a somewhat log cabin-style character.

It was designed by the NSW Public Works Department under the direction of one of Australia's leading water supply engineers, E.M. de Burgh.

The finely detailed dam structures of large-scaled concrete in Art Deco-style sit within a broader landscape setting of outstanding scenic quality.

[1] Nepean Dam was listed on the New South Wales State Heritage Register on 18 November 1999 having satisfied the following criteria.

The potential of the Upper Nepean Catchment Area to supply water was fully developed through the construction of four major dams between 1903 and 1936.

[1] The place is important in demonstrating aesthetic characteristics and/or a high degree of creative or technical achievement in New South Wales.

Upstream of the dam wall this setting is characterised by the broad expanse of the pool of water bordered by the crests of the valley sides.

Downstream of the dam wall the setting is characterised by the steeper inclines that graduate into the gorge created by the river's flow over time.

Collectively this topography at time of high water level imparts a picturesque scene when viewed from select vantage points above and on the dam wall.

[1] The place has potential to yield information that will contribute to an understanding of the cultural or natural history of New South Wales.

[1] The hillside near the upper picnic area was the site of the original construction township and retains an intact road formation, kerbing and guttering and tennis courts from that era.

[1] The place possesses uncommon, rare or endangered aspects of the cultural or natural history of New South Wales.

The crest and valve houses and inlet works retain original ironwork which represent a substantial repository of water supply delivery technology of the era.

[1] The remnant platforms and terraces of the construction area are the largest in scale and most intact in regard to finishes and planning of all four of the Metropolitan Dams.

The reservoir of the Nepean Dam is the second largest source of water in the whole of the network administered by SCA for the supply of metropolitan Sydney.

Key representative e attributes of the dam's design and construction include the use of cyclopean masonry bedded in sandstone concrete, use of blue metal concrete in the wall facings, use of a spillway set away from the gravity wall, valve and crest houses attractively designed and finished to a high standard, the use of an array of upstream intakes to regulate the quality of water supply, the internal inspection gallery, the foundation drainage system, the contraction joints, and the internal drainage system.

Key representative attributes include the use of cableways, the building of temporary camps to house labourers and tradesman, building of semi-permanent cottages to house salaried staff, the construction of terraced platforms for plant and machinery, mechanisation of concrete production, the construction of a purpose-built road of access and tramway to transport men, supplies and materials from the nearest railhead to the construction site, the building of permanent infrastructure such as water supply for plant and men and horses, and the use of electricity to power plant and equipment.