Cosseys Creek Dam

Cosseys Creek Dam, completed in 1955, was the first of a series of rolled earth water supply dams in Auckland's Hunua Ranges. Rolled earth dam design in New Zealand was significantly advanced by these dams constructed between 1950 and 1977.

Cosseys Creek Dam is situated in a young gorge valley about 2.5km upstream of the Hunua Falls. At this point the stream was originally incised in its gorge about 62m upstream of the site. At the time of construction, the valley was in its original mature state with very favourable storage conditions.

The dam was built using rolled fill consisting entirely of residual clays and rubbles from greywacke-argillite rock. All materials were existing and within a haul distance of 5km. The clays and clay-rubble for the core and rock rubbles at the faces were easily selected at the borrow area, and the core construction readily controlled by the use of standard moisture content and compaction density checks. Use of the newly introduced Euclid T24 earthmovers, and tractor-drawn sheep’s-foot rollers made for speedy completion – 411,000 cubic metres of material in 190 placing days over three seasons.

Cossey's Dam on the headwater of the Wairoa River, Hunua Ranges, Auckland Region [12 February 1965]. Whites Aviation Ltd: Photographs. Ref: WA-63447-F. Alexander Turnbull Library, Wellington, New Zealand. http://natlib.govt.nz/records/22337143.

Diversion of the stream took place during construction through a 4.3m tunnel, 235m long, which was subsequently sealed with the valve tower and its five intakes. A bellmouth type spillway feeds to a spillway tunnel discharging to a lined channel below the downstream toe of the dam. The spillway capacity is 15,000 cusecs (441 cumecs).

Filling of the reservoir commenced on 13 May 1955, and was completed by 12 July 1956 following heavier than normal rainfall.

Cossey's Dam, March 1956. Sir George Grey Special Collections, Auckland Libraries, 1207-930.

Later upgrade work

The dam was monitored from the 1970s onwards. Observations in the late 1990s revealed a problem with a restricted drain under the downstream shoulder. This prompted a major reconstruction in 2003-04. The dam was built without a filter or drain zone downstream of the core. Much of the dam was removed in order to reconstruct the downstream shoulder and install modern zoning in the dam. URS (now Aecom) New Zealand undertook a detailed risk assessment and options study, followed by the design of the upgrade and overseeing of remedial works. McConnel Dowell were the contractors.

Other interventions have included strengthening the valve tower, an improved release system and a micro power plant utilising energy dissipated through a flow control valve.

Dam details

Crest length: 168m

Height above foundation: 41.2m

Catchment area: 2,200 ha

Safe yield: 40,800 cu.m./day

Reservoir area when full: 123 ha

Storage volume: 14,030,000 cu.m.

More information

Access

The dam and reservoir can be seen from the Cosseys-Massey Loop walk. Access information is available on the Auckland Council website.

References

“Cosseys Dam Upgrade & Remedial Works”, McConnell Dowell, accessed October 2025, https://www.mcconnelldowell.com/projects/cosseys-dam-upgrade-remedial-works

Wal McQuarrie, “Reflections on the Cosseys Dam Upgrade (2003)”, in New Zealand Society on Large Dams Newsletter, December 2020: 12-14, accessed 08 December 2025, https://nzsold.org.nz/wp-content/uploads/2020/12/NZSOLD-December-2020-Newsletter.pdf.