We operate 12 water supply lakes, five of them located northwest of Auckland in the Waitākere Ranges, two near Helensville, four to the southeast in the Hūnua Ranges, and one east of Papakura. Combined the lakes are capable of storing 95.5 million cubic metres of water.
- Normally the dams supply about 80 per cent of Auckland’s water.
- Because the dams are located at high elevations, water flows easily to the city by gravity.
- Earlier dams in the Waitākeres were made of concrete. After World War II they were built with earth and rock.
- The Hūnua and Waitākere ranges receive about 1.8 metres of rain annually – about 50 per cent more than central Auckland.
The water is piped to Ardmore Water Treatment Plant. It is then stored and carried through pipes, which are up to 1.9 metres in diameter, into reservoirs nearer the city.
- 5 earth dams
- Supply: Normally about 60 per cent of Auckland’s water
- Built: 1951-1977
- Location: Headwaters of the Hūnua River. Cosseys Dam is on the headwater of the Wairoa River.
Raw water from these dams is piped to treatment plants in Huia, Titirangi and Swanson.
- 5 dams - 3 concrete, 2 earth
- Supply: Normally about 20 per cent of Auckland's water.
- Built: 1907-1971
- Each dam is named after the stream that feeds into its reservoir.
Raw water from these two sources is transported to Helensville Water Treatment Plant which supplies treated water to Helensville and Parakai.
There is a common misconception that our dams contribute to flooding – in fact, they do quite the opposite. When they do fill up and spill, the water flow rate over the spillway is slightly less than what would be flowing in the stream if the dam wasn’t there. This is because the presence of the dam delays and suppresses peak flows in heavy rainfall.
We don’t have any control over the rate at which water flows from our dams to the stream below when the dams are spilling, as this is driven by rainfall. We do not have ‘release gates’ like hydro-electric dams.
Our dams are operated under various resource consents granted by either Auckland Council or Waikato Regional Council. One of the consent requirements is to release a small flow of water from our dams to protect the downstream flora and fauna. We call this a ‘compensation flow’. The volume of this flow is very low – designed to keep the stream healthy, and maintain the stream habitat, by providing a base flow when our dams are not spilling.
We are also required to carry out periodic flushing operations at our dams in summer using a ‘free discharge’ valve. This again promotes the health of the downstream environment. Typically, we run these flushing flows for about three hours, and the free discharge valve is only partially opened. The flow rate released during these operations is well below what would be considered a ‘flood’ flow.
Releasing water for critical maintenance
We are also authorised to release water from our dams for operational and maintenance purposes, for example to undertake renewal work on the dam that requires a lower water level to get the job done safely.
When we need to draw down lake levels for dam maintenance, we release water slowly over several weeks. In line with dam safety guidelines, we test our free discharge valves once a year by opening them fully for a short period.
When we need to release water for testing or maintenance, we follow dam safety guidelines and the conditions of the dam’s resource consent, which include considerations like the maximum drawdown rate. We also notify the council and any stakeholders downstream.