The public will get their first look inside the hydro-electric power station at the Clyde Dam in central Otago on Sunday.
For the first time in decades, the doors will be open to the community.
Hundreds of properties were submerged by the dam to create the new power station when Lake Dunstan was created 25 years ago by flooding a large part of Cromwell.
The multibillion-dollar project took 12 years to complete after the lake creation suffered massive budget blowouts - three years was spent digging tunnels in the hills to stabilise landslides.
"It's a huge project. And it was back in the days of the Government's 'Think Big' projects," says project manager Neil Gillespie.
"[There was] a lot of public controversy and in the end it required legislation to make it happen."
The controversy was over the loss of Cromwell's old town along with orchards in the Cromwell Gorge which made way for the massive dam.
The Clutha River was diverted while the dam was being built. Once completed, it was then closed off to create Lake Dunstan.
Greg Foster was the man who pushed the "go" button on the new hydro-electric power station 25 years ago.
"They knew that the growth in demand in New Zealand at the time that this place would be needed. And right now we produce about 6 percent of what the country needs," he says.
During construction, it was found the dam was being built on an active faultline.
It was redesigned as two dams, separated by a concrete wedge, to allow for seismic movement.
Despite all the dramas, Foster believes history now looks more kindly on the project.
"It's a pretty fantastic thing. You've only got to look at the growth in the likes of Cromwell holiday homes, there's hundreds of boats on the lake through the summer, it's incredible. Good fishing, people love it."