Inside Arapuni hydro station’s long high-ceilinged turbine hall with its eight vertical turbine generator units
Ghosts, unexplained noises and a century of history. Mary Anne Gill toured the grand old Arapuni Dam and met the man who keeps it all running.
Mercury station production technician Craig Russell monitors Arapuni Dam inside the hydro station where unexplained noises are often heard. Photo: Mary Anne Gill
There is something about Arapuni Dam that stays with you.
It might be the view from the top, looking out across the Waikato River. It might be the sense of scale when you start to understand what is hidden beneath the concrete.
Or it might be the stories.
Some of them are stranger than others.
Production technician Craig Russell says parts of the station are believed to be haunted and speaks of noises that cannot easily be explained.
It adds another layer to a place that already feels steeped in history.
Inside Arapuni hydro station’s long high-ceilinged turbine hall with its eight vertical turbine generator units – four installed between 1929 and 1932, two in 1938 and two in 1946. Units five to eight, shown in the foreground, were upgraded in 2012. Photo: Mary Anne Gill
Alongside the engineering and the past, there are people who have worked here for years who will tell you this place has its own character.
The dam and power station at Arapuni have been part of the Waikato landscape since 1929. Constructed between 1924 and 1929, it was the first government-built high dam on the river and remains an important part of the hydro system today.
Karāpiro, about 15 kilometres *downstream from Arapuni along the Waikato River, is part of the same hydro chain. Between the two once stood the Horahora power station, built in 1913 and later swallowed by the lake when Karāpiro was created in 1947.
Arapuni can generate up to 806 gigawatt hours of electricity a year, enough to power around 100,000 homes. But keeping a structure like this running is an ongoing job.
Mercury programme manager Tom Newson points to historic photos. Photo: Mary Anne Gill
Mercury is using Skywork Helicopters to resupply its team working on the relief drainage programme at the bottom of the dam.
Tom Newson, Mercury’s programme manager for the current upgrade, said the work never really stops.
“What we have concluded is that those seepage treatments in the left abutment and the retrofits that were done in the 1930s have effectively reached the end of their life,” he said.
That is the reason for the latest project, which will see a new underground cut-off wall installed deep into the ground to manage water seepage.
The work will take between two and three years and involves highly specialised techniques such as jet grouting, where soil is mixed with cement to stabilise the ground before construction begins.
It is complex work, but it builds on a long history of problem solving at the site.
Engineers discovered water leaking through the left abutment soon after the dam was first filled in 1930. Fixes were put in place, and more work followed over the decades as engineers monitored how the dam performed.
In the mid-2000s, a major programme involved installing cut-off walls beneath the dam. That work has held up well, but changes in groundwater pressure were identified after distant earthquakes in 2016.
Newson said monitoring is constant.
“We’ve got about 120 real-time logged instruments in the abutment measuring groundwater pressures and seepage flows,” he said.
That level of attention reflects how the dam is operated.
“It’s a live patient,” he said. “We’re monitoring all the vitals as we’re performing the works in real time.”
A view looking north towards Putāruru of the hydro dam during the jet grouting trial, winter 2025.
One of the striking things about Arapuni is how the old and the new sit together.
Newson compares it to something familiar.
“I kind of liken it to grandfather’s axe,” he said.
“You replace the handle a few times, you might replace the head, but it’s still grandfather’s axe.”
That idea of continuity runs through the site.
The original dam was built by the Public Works Department using the tools and techniques of the time. Photographs from the era show workers standing high on structures with little in the way of modern safety gear.
A tunnel was constructed to divert the Waikato River so work could begin on building the dam.
The dam was constructed between 1924 and 1929.
Today’s engineers are working with advanced monitoring systems, international contractors and specialised drilling rigs.
But in both cases, the aim is the same. Keep the dam working.
The work ahead will focus on the left side of the dam, where a new cut-off wall will extend tens of metres down into the ground to limit water seepage.
Before that can happen, the ground itself has to be strengthened.
Jet grouting will be used to create columns of stabilised soil, forming a base for the wall and helping prevent collapse during drilling.
All of this will happen while the dam continues to operate.
As Newson points out, Arapuni is only one part of a larger system.
“It’s part of a cascade system, it’s a very important link,” he said.
That makes shutting it down for long periods difficult, so the work is carefully staged to allow generation to continue.
The human side of the story is never far away.
Staff on site have their own history with the dam, and their own stories.
There is the climb known as the Stairway to Heaven, a steep set of 237 steps leading up toward the dam.
And then there are the stories that get passed around.
The records room is a case in point. Inside it are maintenance logs for every day of the station’s life.
Project manager Murray Ballantyne holds a record book open at the notes made the day senior writer Mary Anne Gill was born in nearby Putāruru.
The records room in Arapuni Dam’s hydro station with boxes of decades-old maintenance notes. Photo: Mary Anne Gill
Among them are entries from the Saturday I was born in nearby Putāruru. They show station operators keeping the turbines running smoothly by managing electrical switching, monitoring pressure and temperature, and controlling cooling pumps. Alarms were tested, filters cleaned and equipment adjusted as needed. Despite minor issues, the station ran steadily throughout the day.
It is a reminder that while lives begin nearby, the dam just keeps going.
For Mercury, the focus is firmly on the future.
The current project is a significant investment aimed at extending the life of the dam and supporting the wider hydro system.
“It’s about maintaining and operating a sustainable asset well into the future,” Newson said.
The dam will mark its 100th year soon, a milestone that highlights both how long it has been part of the landscape and how much work has gone into keeping it that way.
Standing on the dam today, it is easy to see why it matters.
It is not just a piece of infrastructure.
It is a place where engineering, history and everyday stories all come together.
See: Arapuni dam road to close
*the print and earlier online versions said upstream. It is downstream.
Before the Nazis adopted the swastika symbol, it was used by the Swedish electrical company ASEA (Allmänna Svenska Elektriska Aktiebolaget) from the late 1800s until 1933. The ancient symbol stood for good luck, prosperity and the sun and can be seen inside Arapuni hydro station on four of the dam’s turbines. Photo: Mary Anne Gill
Waygood-Otis (now Otis) Lift Number 66684 was built to carry 6000 pounds when installed into the hydro station. It still works today. Photo: Mary Anne Gill
Arapuni Hydro Station opened in 1929. Photo: Mary Anne Gill
The Arapuni Hydro Station today, with the swing bridge above.
Inside Arapuni hydro station’s long high-ceilinged turbine hall with its eight vertical turbine generator units – four installed between 1929 and 1932, two in 1938 and two in 1946. Units five to eight, shown in the foreground, were upgraded in 2012. Photo: Mary Anne Gill
