The $220m expansion at Mercury’s Ngā Tamariki Geothermal Station. Pictured on-site are (from left) mechanical project engineer Lexi Richards, programme manager for major geothermal projects – generation Aimee McGregor, and head of strategy – generation Matt Kedian. Photo / Megan Wilson
The $220m expansion at Mercury’s Ngā Tamariki Geothermal Station. Pictured on-site are (from left) mechanical project engineer Lexi Richards, programme manager for major geothermal projects – generation Aimee McGregor, and head of strategy – generation Matt Kedian. Photo / Megan Wilson
A $220 million expansion of Mercury’s Ngā Tamariki Geothermal Station in Reporoa near Taupō is “90%” complete and will be generating electricity by the new year.
The project – which started in early 2024 – involves adding a fifth electricity generation unit to the station, called OEC5 (Ormat Energy Converter).
The expansion will add a further 46 megawatts to the station, moving its installed capacity from 86MW to 132MW.
This will increase its annual average generation output by 390 gigawatt hours – the equivalent of generating enough power for about 55,000 homes, the same as the number of residential homes in Tauranga.
Once the upgrade is complete, the total annual average generation output will be 1120GWh – enough power for 158,000 average homes, more than the number of residential homes in Christchurch.
The Rotorua Daily Post was invited to visit the OEC5 site this week.
Mercury’s programme manager for major geothermal projects – generation, Aimee McGregor, said the Ngā Tamariki Geothermal Station used geothermal to make electricity through a “binary” method.
This involved using geothermal fluid to heat a second fluid called pentane. It would constantly heat and cool the pentane, which drove the turbine.
The turbine turned a shaft connected to an electrical generator.
McGregor said the commissioning phase for OEC5 had just started. This meant the team was satisfied everything was working and it was ready to bring pentane into the system.
She said OEC5 would be generating in December and “running smoothly” early in the new year.
Construction of the fifth generation unit at Mercury’s Ngā Tamariki Geothermal Station in May 2024. Photo / Mercury Geospatial Team
Construction of the fifth generation unit at Mercury’s Ngā Tamariki Geothermal Station in August 2025. Photo / Mercury Geospatial Team
McGregor said that historically, the process would involve a lot of carbon dioxide coming out of the ground.
“We now have technology where we are actually able to keep the CO2, mix it in with the fluid that we’re putting back in the ground, and inject it, so it’s much better from a sustainability greenhouse gas perspective.”
The draft strategy paper said “supercritical” geothermal technology – which involved drilling deeper into the Earth’s crust – could offer up to three times more energy than current geothermal energy.
The Government ring-fenced $60 million from the Regional Infrastructure Fund to fund research into supercritical geothermal technology.
Mercury’s head of strategy – generation, Matt Kedian, said its geothermal fluid temperature was between 200C and 300C.
Ngā Tamariki’s geothermal fluid temperature was about 290C, and the reservoir was about 2200m straight down, he said.
Supercritical was above 400C and “at high pressure”. This was usually “well beyond” four to five kilometres straight down, he said.
“So, you’ve got a very, very dense energy source deep in the Earth.
“It’s a really exciting prospect if you could find a way to harness that energy. There’s great opportunity for even more and more geothermal.”
He said a number of countries, organisations, and companies were trying to find ways to “sustainably harvest supercritical”.
