there was an extensive narrow gauge railway along the coast of New Zealand installed only to remove kauri lumber, and during the removal, gold reefs were discovered by accident when using "driving dams" to flood canyons and float out the logs

I came across this logging railroad photo on a Russian blog, and looked for a cleaner (this one) image than the fuzzy overly reposted image they featured.

the locomotive engine was called the Sandfly

Then I learned all about the rare huge trees in New Zealand that were on the remote coast, that a Dr bought the forest range containing them, and then spent 30 years trying to get the timber out.

It was so difficult, that they tried every thing, even driving dams (flood a canyon, use it to float the logs, then burst dam after dam along the way to float the logs to the ocean in stages.... a lot of log jams, not much progress!

in the 1800s the main interest in Piha were the fine stands of kauri. The problem, however, was that much of it, even if harvested, would be difficult or near impossible to transport, first out of the forest, then to the sawmill in Karekare, some eight kilometers away. Once in Karekare, the wood would still need to be transported down the coast another eight kilometers or so to where ships could pick up and transport the lumber.

In 1860s the kauri forest in Piha Valley was bought by a Dr. Stockwell, and for twenty years he unsuccessfully tried to log and transport the kauri out of the remote valley. Once he had gotten the logs out of the forest, part of this plan entailed strapping the logs to rafts, dragging the rafts out to sea and then down the coast. His efforts failed. Thirty years passed.

Finally the problems were solved by building a sawmill in Piha—processed lumber was far easier to transport than kauri logs—and a steam powered tramway was constructed that lifted lumber out of the Piha valley up 300 meters with a grade of 1:1.45 (22%) and then lowered it down a grade to Karekare, a grade of 1:2.5 (36%), over a total distance of about 8 kilometers.

https://commons.wikimedia.org/wiki/Category:W._G._Bagnall_locomotives
https://movin2newzealand.wordpress.com/2015/09/30/the-piha-tramway-piha-karekare-and-whatipu/
https://ritkanlathatotortenelem.blog.hu/2018/11/04/napi_erdekes_571

In 1904, Smyth Bros. found gold in one of their driving streams, Omoho Creek, exposed by the action of the floating kauri logs about 4 miles north of the Royal Oak Mine. By driving timber down the Omoho Creek towards Kennedy Bay the creek channel had been scoured clean, thus revealing many fine reefs and leaders. The leader discovered crossed the creek in north-southerly direction. Strong gold was visible in the stone for over 30 feet, the size of the reef being up to 6 inches in thickness, so that an experienced prospector was contracted to retrieve the gold

https://en.wikipedia.org/wiki/Messrs._Smyth_Brothers%27_Tramway