The Tokaanu Power Station is part of the Tongariro Power Scheme. When the power station was built, the tail race taking water from the turbine hall to the lake needed to cross paths with an existing natural trout spawning stream.
To solve this problem and allow trout to still access the upper reaches of the stream, the stream was artificially diverted about 200m to the north and this double level bridge built carrying the stream across the tail race, beneath the road deck.
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At the published coordinates you will find 1972 with some other numbers below. You will use those numbers to find the final geocache by performing some pretty easy calculations.
Take AAA and BBB from the published coordinates where AAA is the final three digits of the published South coordinate, and BBB is the final three digits of East: S 38° 58.AAA' E 175° 46.BBB'
The final cache is located at S 38° 58.XXX' E 175° 46.YYY' where:
XXX = AAA - (H x 5) + (S + T)
YYY = BBB + (H x 5) - (S x T)
Now you have the final coordinates, go find the cache! You're looking for a small screw top container held in place by invisible force. Please use stealth as best you can, and hide the cache all the way back inside it's hiding spot.
BYO pen, happy caching, and good luck!