ARAPUNI DAM REMEDIATION WORK
The Arapuni Hydro Dam The Arapuni Hydro Dam is operated by Mercury and forms an essential part of the Waikato Hydro System, which provides about 10% of Aotearoa New Zealand’s electricity.
The Arapuni Hydro Dam and nearby Arapuni Hydro Station began operating in 1929. As the first high dam on the Waikato River, it is recognised for its engineering significance and is a listed historic heritage item. The hydro station’s eight turbines have an installed capacity of 196 MW, generating up to 805 GWH per year, that’s enough to power about 100,000 average homes with renewable energy. Project overview
An engineering study shows the Arapuni Hydro Dam is safe and fit for everyday use, but work needs to be done to improve the left abutment to protect it from the risk of water seepage. Seepage happens when very small amounts of water soak around the dam abutment. A controlled amount is expected and needs to be managed, but over time too much seepage will result in increased pressures and a risk of internal erosion.
We are assessing options to install a seepage cutoff wall in the left abutment, this is a concrete barrier that prevents seepage. We are working on geotechnical assessments which will help inform the remediation work design and construction.
As part of this process, we’ve begun discussions with our iwi partners, Raukawa, Ngāti Korokī Kahukura and Ngāti Hauā, as well as the South Waikato District Council, Waikato Regional Council, Waipā District Council and other key stakeholders to better understand the impacts of the remediation work. We will keep the community informed as plans are developed. Project background
The dam has had remediation treatment for leakage, dating back to its construction. Following the first lake fill in 1930, significant seepage of water traveling through the left abutment from the headrace to the dam face was observed. The lake was lowered, and seepage controls were retrofitted.
The Arapuni Hydro Power Station was constructed between 1924 and 1929.
Major remediation work was completed on the dam in the mid-2000s.
Mid-2000s works
Over the life of the dam there have been several improvements made to the seepage controls, with the most recent in mid-2005.
This involved remediation work to construct a concrete cut-off wall under the dam, without disrupting power generation or causing environmental damage. This was a pioneering effort in the field of dam engineering.
The project won multiple engineering awards, including the Shell Environmental Excellence Merit Award in 2007 and the New Zealand Engineering Excellence Award in 2008. The work was recognised for its innovative approach to dam remediation while maintaining operations and minimising environmental impact.
The project was led by an alliance group comprising Mighty River Power (now Mercury), Brian Perry Civil and Trevi Group, a specialist in underground engineering. The three have reformed to work together on the remediation of the left abutment at the Arapuni Dam.
The left abutment of the dam is where our current remediation work will be focused.
Current early works
Ongoing monitoring indicates that the remediation work completed in the mid-2000s was extremely effective. However, seepage changes were identified in the left abutment following two distant earthquakes in 2016.
In 2024, we began investigating options to install a seepage cutoff wall in the left abutment, as a modern equivalent to manage the seepage.
We began early works in 2025, relocating old and installing new subsurface instruments to help us understand what’s happening under the left abutment. We now have more than 100 subsurface sensors providing real-time data. This information will help us respond to any changes in the left abutment as we carry out the remediation work.
Jet grouting trial
In mid-July 2025 we plan to run a jet grouting trial. This is a soil improvement technique used to strengthen and stabilise the ground. It has been used in several hydro dam remediation projects around the world.
Our jet grouting trial will take three weeks and be followed by a programme of verification tests over the following four-six weeks. During this time people will see a specialised jet grouting rig on site, slightly larger than the one we had been using earlier this year for the investigation and instrumentation drilling of the new subsurface sensors.
We will be using traffic management to help us move the rig to a few different locations to complete the trial.
Download our info sheet on jet grouting here.
Relief drainage
Hydro dams hold back large volumes of water, which exerts significant pressure on the dam structure and the surrounding geology. Relief drainage is used to help reduce pore water pressure in the dam foundation and abutments and prevent uplift pressure that could destabilise the dam base. In August, we will lower another drill rig to the toe of the dam to begin relief drainage drilling over five-six months.
Main works
The jet grouting trial results will help us develop plans for the main remediation works. As we develop plans, we will continue to engage with partners, stakeholders, and community to let them know what is happening.
We expect to submit a business case to our board for approval in late 2025. This will determine the timeline for design and how we progress to the construction phase.
Once the business case is approved, design and construction are estimated to take 18-26 months to complete. Timeline
Map
Download map here.
Project milestones
2025
2024
December 2024 newsletter April 2025 newsletter July 2025 newsletter
From mid-2025, we will be working on relief drainage at the bottom of the dam. We will also be working at the top of the dam, using a rig to relocate and introduce more subsurface instruments. This will help us monitor what’s happening underneath the left abutment.
From mid-2025, we will also be working on a jet grouting trial. The results of all of this work will help inform the main programme of work which we’re aiming to start in 2026.
In 2025, we may need to use temporary traffic management on Arapuni Road to move equipment, but the road will remain open throughout this stage of the project.
In 2026, Arapuni Road across the dam will be closed to allow us to complete the main work on the abutment. We will use a one-lane detour route, managed by traffic lights, along Power Station Road which reconnects to Arapuni Road, north of the village. More details for the closure timing and durantion will be provided once understood.
For safety, we will have to close the western end of the swing bridge which connects to Power Station Road, where people walking and cycling would normally join the road.
We are still developing options for the remediation work; we will complete technical assessments to better understand any impact on the local community. As we start to develop plans for the works, we will engage with the community to see how we can minimise disruptions.
We held our second community drop-in event on 12 April 2025 and we are planning our third event in September 2025.
While there will be no change to the electricity generation, doing the required works now ensures that future generations can continue to benefit from this integral hydro asset. The power station will continue to operate as normally during the work.
We want to better understand what’s important to local people and the groups we work with. As we start to develop plans for the remediation work, we will continue to engage with stakeholders and local community to let them know what is planned. If you would like to contact the project team, please email: project.arapuni@mercury.co.nz.
Our contractors, Avalon, began work on clearing vegetation from the left abutment in October 2024 and completed the work in November 2024. We needed to clear the vegetation so we could accurately geo-map the abutment to determine the best plan for relief drilling work, which we want to start in early 2025.