ARAPUNI DAM ENHANCEMENT PROJECT
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 196MW, generating up to 805GWH per year, that’s enough to power about 100,000 average homes with renewable energy.
Explore general information about the Arapuni Hydro Station.
The jet grouting trial work site was set up on the left abutment from July-September, at the Arapuni Hydro Dam.
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 have assessed options to install a seepage cutoff wall in the left abutment; this is a concrete barrier that prevents seepage. We have also been working on geotechnical assessments which will help inform the work design and construction of the cutoff wall.
Since mid-2024, we’ve been in discussions with our iwi partners, Raukawa, Ngaati Koroki Kahukura and Ngaati Hauaa, as well as the South Waikato District Council, Waikato Regional Council, Waipā District Council and other key stakeholders to understand the impacts of the work we have planned for the dam. We have kept the community informed as we developed our plans and we will continue to update everyone as the project progresses.
Video: Arapuni Project update No. 1
Video: Working with Iwi
Project background
The Arapuni Dam is the oldest Mercury dam (in operation). It was built between 1924 and 1929 and was the first government-built high dam on the Waikato River.
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-2000s.
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.
Early works, 2024-2025
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.
In 2025, we began early works which involved relocating existing subsurface sensors and installing new ones to help us understand what’s happening in the left abutment. We now have more than 200 subsurface sensors providing real-time data. This information will help us respond to any changes in the left abutment as we carry out the main works programme.
Jet grouting trial
We completed a jet grouting trial from July to September 2025.
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. During September and October, we studied the results of the trial which helped inform and plan the main works for 2026.
Download our info sheet on jet grouting.
Relief drainage
We started a relief drainage programme in January 2026 and will complete it by July 2026.
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.
Watch our video on the relief drainage programme.
Detour route
Arapuni Road across the dam will be closed to vehicles from July 2026, for 24-36 months.
We will use a one-lane detour route, managed by traffic lights, along Powerhouse Road for local essential traffic only.
Download the Powerhouse Road detour route map for essential travel (PDF)
Freight/heavy commercial vehicles and long-distance travel should use state highway routes.
Download the alternative route for heavy vehicles (PDF)
For safety, we will close the western end of the swing bridge where it connects to Powerhouse Road.
We are making changes to Powerhouse Road so it is safe for Waikato River Trail users and safe to use as a detour route when the main works start.
Main works
We submitted a business case to our board which was approved in late 2025. We have finalised the main works timeline and we are progressing towards the construction phase in mid-2026.
We expect construction to take 24-36 months.
Timeline
Project milestones
2026
- In April we held our fourth community drop-in session at Rhubarb Cafe, to update people on our plans heading towards the main works phase of the project.
- In early March we began upgrades to Powerhouse Road, between Arapuni Road intersection and the lower bridge across the Waikato River, to prepare it for use as a temporary detour route.
- We hosted the South Waikato District Council at the dam site and led them on a tour of the Arapuni Hydro Station, to bring them up to speed on the project.
- We submitted a business case for the main works programme to the Mercury board.
- In February our relief drainage work started to make good progress, on track for completion in July.
- In January we began setting up a new work site at the bottom of the dam for our relief drainage work.
2025
- We reset the project site in November to prepare for the start of our relief drainage work at the bottom of the dam.
- We held our third community drop-in session at the Rhubarb Cafe on 4 October.
- Our iwi partners from Ngāti Koroki Kahukura became involved in the management of the jet grouting trial site and environmental monitoring.
- Rock slope stablisation work continued at the left abutment and near the track leading to the toe of the dam.
- Members of Mercury’s Board of Directors visited the work site to learn more about the project.
- Road and traffic experts surveyed the proposed detour route.
- Our jet grouting trial ran from July-September at the crest of the dam.
- We held our second drop-in session for the community at Rhubarb Cafe in April.
- We cleared vegetation to make it easier for drivers to see each side of the dam as they enter the work site area.
- We were taken through a cultural induction event by Ngāti Koroki Kahukura from Pōhara Marae and Maungatautari Marae to allow us to start our drilling work on the left abutment of the dam.
- We established the first of two drilling rigs on site for relocating and installing new underground sensors.
2024
- We began our ongoing engagement with partners, stakeholders and the wider community in mid-2024.
- We’ve been working closely with local iwi, Raukawa and Ngāti Korokī Kahukura to better understand the impacts of the remediation work.
- We met with Waikato Regional Council, South Waikato District Council and Waipā District Council staff and elected members to share information about the project, in September and October 2024.
- We held a drop-in session at Rhubarb Cafe, Arapuni, to speak one-on-one with local people so they could learn more about the project, on 5 October 2024.
- Some of our staff and contractors attended a cultural induction hui led by kaumātua at Pōhara Marae, on 16 October 2024.
- The induction cleared the way for us to begin work on removing vegetation from the left abutment of the dam, from October-November 2024.
- Clearing the vegetation allows us to undertake geomapping of the abutment so we can plan for relief drilling work in early 2025.
NEWSLETTERS
Frequently Asked Questions
We are working on the final design details for constructing an underground, seepage cutoff wall in the left abutment of the dam. This is a concrete barrier that will help protect the dam from the risk of water seepage.
We have been focusing on the enabling works since mid-2024, to verify the methods and technical assessments to install the cutoff wall. We are now preparing the dam for the main works programme which will start by June/July 2026 and operate for 24-36 months.
Yes. The road across the dam will be closed for 24-36 months so we can safely work on the left abutment. We will have a team of contractors and equipment moving around the work site, so keeping everyone safe is paramount. We aim to work during the day and then possibly at night so we can get the job done quickly and reopen the road as soon as we can.
Powerhouse Road was successfully used during earlier works in 2005, but the new project is longer and must meet higher modern safety standards. We are working on upgrades so it can be safely used as a detour route during the multi-year timeframe of the project.
The purpose of the detour is to continue to let local communities connect for work purposes, to attend schools, for people who have farms and need to use small vehicles to travel to properties on either side of the dam.
No. Engineering assessments confirmed that Powerhouse Road cannot safely accommodate High Productivity Motor Vehicles or long heavy units, oversized vehicles, and extended freight trucks over a sustained period. Restrictions are required due to the route’s physical constraints, narrow shoulders, tight geometry, gradients, and a one ‑lane traffic‑light‑controlled section that makes it unsafe for HPMVs and long units.
Cars, light vehicles, motorcycles, school buses, emergency services, rigid trucks up to 8m.
HPMVs and large freight will need to use the State Highway network (SH1/SH21/SH3 via Tamahere) if they wish to travel from east to west and vice versa without going through Arapuni.
No. The detour is not safe for walkers or cyclists to use with vehicle traffic. We’re working with Waikato River Trails to make sure users are aware of the closure.
If more cyclists arrive expecting to cross the dam or use the detour, we’ll step up communications and look at other ways to share this information. In situations where it’s necessary and feasible, we may use our site vehicles to transport bicycles using bike racks. However, this is not intended to become a regular solution, as we are not providing a shuttle service.
No. To keep everyone safe, we will close the western end of the swing bridge where it connects to Powerhouse Road. Pedestrians and cyclists won’t be able to use the walking trails near Powerhouse Road, but the Waikato River Trail will remain open and available for people to use on the village side of the river.
Engineering advice suggests keeping this open is an unacceptable risk to public safety, previously in the mid-2000s we experienced vehicles stopping in the single lane section with the swing bridge was open that caused significant operational and safety impacts to the detour.
Yes. The cafe will continue operating as usual, and any changes to their hours will be made solely at their discretion and communicated through their own channels.
To support visibility throughout the detour period, we have worked with the cafe to create roadside signage clearly signaling that the cafe remains open. These signs will be installed shortly before the detour becomes operational to ensure road users know the cafe is still open.
There will be limited opportunities for parking and to fish from the headrace bridge.
All other locations, such as tailrace bridge and dam, will be restricted during construction with the detour in place, as these areas will be closed for safety. Clear signage will be installed to indicate restricted access, and our on‑site team will be available to provide guidance if needed. Sightseeing around the dam will also be affected, particularly for those who usually walk through the dam area or use the car park. These areas will be cordoned off for construction activity and access changes. We will communicate any restrictions or updates early and clearly so people are aware.
Yes. We know fuel prices are high and that detours add time and cost. This detour has been designed to minimise delays (around 10–12 minutes), and we encourage people to plan ahead using tools like the NZTA Journey Planner and council and NZTA updates to find suitable routes. While fuel prices are outside the project’s control and increased after traffic planning was completed, the detour is essential to deliver this work safely and without significantly extending the project. We will continue to provide timely updates so people can plan their travel and move safely and efficiently.
We plan to meet again with the community later in 2026, after we start the main works of the project.
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 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.