It was meant to be a routine photo opportunity: government minister presses the detonator plunger, the blast goes off and the Manapouri power station's tailrace tunnel arm is officially completed.
The real hole-through - the explosion connecting two ends of the tunnel - had happened about 11 hours earlier, around midnight. This was a ceremonial re-enactment for the cameras, on January 4, 1969.
Except the blast misfired. The concussion sent safety hats flying in the air, like a scene from a Looney Tunes' Road Runner skit. Dominion photographer Barry Durrant caught the cartoon moment.
Recalls Durrant: "This tremendous concussion came out along the tunnel, all the lights went off and all you could hear was little pieces of rock fragments. The shockwave came roaring down the tunnel after the bang, like a huge gust of wind - more than enough to give you a good push on the back.
"We were all stunned - no one knew it was going to be such a big blast; there was deadly silence. Everyone got a bit of shock. Then someone turned on a torch, and we all laughed and clapped."
No one was hurt. But the camera belonging to a local photographer, in the background of this picture, never worked again.
Durrant, who was working on contract for the construction company, had been allowed to photograph the real hole-through. So, while the other photographers snapped Justice Minister Ralph Hanan pressing the plunger, only Durrant knew to wait for the concussion.
It would be three years before the wider project, the vast underground hydroelectric station, would be complete.
But this was a significant milestone in a contentious project which was designed to power the Bluff aluminum smelter. Opposition to the power station galvanised New Zealand's emergent environmental movement.
In all, 3300 tonnes of explosives were used in the station's construction.
A re-enactment for the cameras gained a comical twist when it misfired - one of many explosions, both literal and metaphorical, in the making of the Southern Hemisphere's largest underground power station.
