Lake Onslow option

Pumped hydro storage at Lake Onslow is one option being explored.

Facts and figures for Lake Onslow project

The Lake Onslow option referenced by the Interim Climate Change Committee could be anticipated to provide at least 5TWh of annual generation/storage. It is estimated to have a construction timeframe of 4-5 years, with commissioning and filling taking a further 2 years. At its construction peak, it is expected to create 3,500-4,500 skilled and semi-skilled jobs.

What services can a pumped storage project like Lake Onslow provide?

  • Dry year storage—we are specifically investigating Lake Onslow given its ability to store up to 5-7TWh for dry year support
  • Intermittency back up—our existing hydro lakes can increase or decrease their output to offset the variation in wind or solar generation, but this capacity is limited. Pumped hydro could provide a form of back-up to ensure electricity supply and demand is met when generation from solar, wind and existing hydro are not enough.
  • Fast response reserve—pumped hydro can potentially provide technical services that ensure system stability in the electricity market.

What is the estimated cost of a project like Lake Onslow?

Early estimates indicate a project like Lake Onslow could cost about $4 billion. Our feasibility study will provide greater certainty about the costs.

How would it be funded?

Funding and financing models along with any potential subsequent levies, will be determined through the feasibility study. The use of a levy would need to be justified via a demonstration of significant public good in a feasibility study, including through overall lower prices for consumers in the long term.

Where is the Lake Onslow option located?

The Lake Onslow option is located north-east of the Clutha River in Central Otago.

The diagram shows what a 5TWh proposed lake would look like at the 760 metre contour. This is not the only possible option for Lake Onslow.

The NZ Battery Project is considering pumped hydro projects at a range of different locations, in addition to alternative technologies.

Map of the 5 TWh/760 metre countour option at Lake Onslow.

Feasibility study

In October 2021, the Energy and Resources Minister announced a significant contract to investigate the engineering, environmental planning and geotechnical feasibility investigations at Lake Onslow had been awarded to Te Rōpū Matatau.

Te Rōpū Matatau is a consortium of firms led by engineering consultancy Mott MacDonald New Zealand, with engineering consultancy GHD and environmental planning and design consultancy Boffa Miskell.

This work, along with the environmental and cultural investigations already underway, will give a better picture of the feasibility and costs of the Lake Onslow storage scheme.

Te Rōpū Matatau's study is broadly divided into 2 overlapping phases: Phase 1A and Phase 1B.

Early field work during Phase 1A  focused on establishing key elements of feasibility around Lake Onslow itself. These elements include information on nearby availability of material for dam construction, geology of the possible dam wall area and better understanding of tunnelling costs.

This early field work has confirmed it appears technically feasible to create a pumped hydro scheme at Lake Onslow. However, more work is needed to analyse potential construction, engineering, cost, timing and consenting, as well as integration with the transmission system.  

Phase 1B of Te Rōpū Matatau’s feasibility study includes further geotechnical and engineering field work to understand the regional geology, rock properties and faulting and will help determine the best possible location and design of pumped hydro infrastructure.

Fieldwork for this phase is underway and includes drilling boreholes to better understand the underlying geology, the best route for a tunnel and the best location for a potential underground powerhouse.

Te Rōpū Matatau’s feasibility study also considers consenting options and what further work may be needed to assess the environmental effects.

The Lake Onslow feasibility study takes into account the technical, environmental, social, cultural and commercial considerations of the scheme and is expected to be completed by the end of 2022. At this point, Ministers will consider the feasibility of all options being investigated and are expected to make decisions about which option or options should be further investigated for Phase 2 of the Project.   

Read the press release from the Minister of Energy and Resources:

Major contract awarded to power NZ Battery investigation(external link) —

Geotechnical investigation

Te Rōpū Matatau was granted resource consents and other permits earlier this year to undertake geotechnical, geological and hydrogeological investigations, to inform the Lake Onslow feasibility study. 

Work began at the first site in mid-April 2022 and continued at various times throughout the year across a number of sites near Lake Onslow, between the lake and Clutha River / Mata-Au, and at various locations in the Teviot Valley area. Work at the final site is expected to be completed by November 2022.

These feasibility geotechnical investigations include drilling for core samples and groundwater monitoring, excavating or digging test pits to observe soil and rock material near the surface, collecting rock samples for testing, and non-intrusive geophysical surveys. 

This work helps build a broad picture of how the regional geology could best support a pumped hydro scheme and provides valuable information to help understand:

  • regional geology such as faults, shear zones and landslides
  • whether rock strength and other properties vary between locations
  • groundwater and surface water interactions
  • suitability for pumped hydro infrastructure at different locations.

Prior to applying for consents, independent experts were contracted to assess the potential effects of the geotechnical work on each site, including ecological, cultural and archaeological effects. Sites were then selected where the impact on these values and on the local community is considered low or can be avoided or mitigated. 

Permission from landowners was sought prior to lodging consents for fieldwork on private land.

Last updated: 22 September 2022