Rotokawa Geothermal System

Location and general description

The Rotokawa Geothermal System is located within the Waikato Region between the Tauhara and Ohaaki geothermal fields, about 10 km northeast of Taupo, and is one of a number of high temperature geothermal fields in the Taupō Volcanic Zone. The associated thermal area has extensive surface features including fumaroles, eruption craters and collapse pits. Lake Rotokawa fills one crater, and there are large deposits of sulphur surrounding and beneath the lake. The field covers an area of about 15 to 18 km².

The Waikato Regional Plan classifies the Rotokawa Geothermal System as a Development Geothermal System where the take, use and discharge of geothermal energy and water will be allowed, while:

• remedying or mitigating significant adverse effects on Significant Geothermal Features; and
• avoiding, remedying or mitigating adverse effects on other natural and physical resources including overlying structures (the built environment).

This classification is based on:

• The system is already subject to large scale energy use and development
• Few surface outflows vigorously depositing sinter.
• No evidence of a flow of subsurface geothermal fluid to or from a Protected Geothermal System.

Local Geology and geophysics, wells drilled, etc.

The energy potential of Rotokawa was recognised in the early 1950's and drilling identified a large high-temperature resource. The Crown drilled six exploration wells over the period 1967-85. Production is from around 2,000 to 2,500m, with temperatures up to 320°C, and fluid is reinjected at depths of 2,500 to 3,100m. By December 2020 more than 30 wells had been drilled at Rotokawa, and currently 12 wells are operating as production wells and 6 wells as injection wells.

The subsurface geology of the Rotokawa Geothermal Field has been interpreted from drill cuttings and cores. Basement Greywacke is overlain by the Rotokawa Andesite, a sequence of andesite lava flows and breccias up to 2,000m thick. Differences in the elevation at which the Basement Greywacke and Rotokawa Andesite are encountered in wells suggest the occurrence of a series of SW-NE striking normal faults, parallel to the structural trend of the Taupo Volcanic Zone, which have resulted in a structural trough (graben) between RK4 and RK6. RK17 is the only well that can be confirmed as having intersected one of these faults.

Overlying the Rotokawa Andesite, and infilling the graben, are the volcaniclastic and sedimentary deposits of the Tahorakuri and Waikora formations (members of the Reporoa Group), which are in turn overlain by the Wairakei Ignimbrite. The elevation of the top of this ignimbrite is relatively constant across the field suggesting no, or very little, reactivation of the graben-forming faults since its deposition. Lack of Wairakei Ignimbrite in RK16-RK18 suggests that the ignimbrite ponded against a fault scarp to the east of these wells. Overlying Wairakei Ignimbrite are the rhyolitic tuffs, ashes and breccias of the Waiora Formation. Haparangi Rhyolite lavas and breccias occur within the Waiora Formation, which is then overlain by mudstones, siltstones and sandstones of the Huka Formation intercalated with the Parariki hydrothermal eruption breccias.

Natural features

Significant Geothermal Feature types present include:

Lake Rotokawa

• Geothermally-influenced aquatic habitat (Lake Rotokawa and Parariki Stream)
• Geothermally-influenced water body (Lake Rotokawa and Parariki Stream)
• Heated ground habitat
• Hydrothermal eruption craters
• Springs vigorously depositing sinter (lake-edge springs).

At Rotokawa North

• Geothermally-influenced aquatic habitat (Parariki Stream)
• Geothermally-influenced water body (Parariki Stream)
• Heated ground habitat
• Hydrothermal eruption craters

At Parariki Stream

• Geothermally-influenced aquatic habitat (Parariki Stream)
• Geothermally-influenced water body (Parariki Stream)

Geothermal vegetation

The area of heated ground and geothermal vegetation surrounding Lake Rotokawa is of national significance because it is comprises a large, relatively good quality area of geothermal vegetation, which includes nationally uncommon habitat types (fumaroles, geothermally heated dry ground, geothermal stream margins). The site also includes a wide diversity of geothermal habitats, and provides habitat for several At Risk plant and animal species. While the site has a long history of modification, the quality of the site is noticeably improving since the Department of Conservation has started managing it.

Existing and historical geothermal uses

The 24 MW(e) Rotokawa power station, an Ormat geothermal combined cycle station, was commissioned in 1997 and subsequently expanded to 34 MW(e) in 2003. The Rotokawa project is divided into two companies; Rotokawa Joint Venture (a 50:50 joint venture between Tauhara North No.2 Trust and Mercury NZ Ltd.) which owns the steam field, and Rotokawa Generation (100% Mercury NZ Ltd.) which owns the generation plant. Mercury NZ Ltd. operates both the station and the steam field. Current annual generation is about 240 GWh.

The Mercury NZ Ltd. / Tauhara North No 2 partnership also developed the 138 MW(e) triple flash plant Nga Awa Purua (NAP) power station, commissioned in 2010, which, after the initial development at Wairakei, is the largest geothermal power station in New Zealand. It also has the world’s largest single geothermal turbine.

The Rotokawa upgrade project was completed in mid-2021 which included a new steam field separator (H-line Separator) to enable brine transfer from the NAP production header to Rotokawa (RKA), and a number of operation upgrades to RKA to handle the increased brine flows.

Power plant details are provided in the following table.

Inferred size of resource

Based on initial stored heat models, the power development potential could be about 270 MW(e).