The downside of hydro electricity is that there's a lot *less* recreational facilities for whitewater kayaking and rafting and tramping. And, in the North Island, a lot of either productive pastures or forest drowned. And, as you point out, loss of habitat for some species which depends on fast-running water. These species, of course, contribute to NZ's reputation as the only place where a whole lot of unique wildlife can be seen, which is of course a major attraction for tourists.
I wonder about the economics; is a billion dollar dam better than 50,000 homes set up for solar?
They submerged two of the world's largest geysers under Lake Ohakuri in 1961.
I wonder if the geysers would still be there if we drained the lake.
Then, easy, now never