Power shares

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Approximately 17m more than current if the lake profile was uniform, in reality you could probably do it with 10m if you could submerge Taupo CBD and Turangi. Then you need to get the water from somewhere. In other words, not possible...

You can't repurpose any of the existing hydro systems to achieve what Onslow would. You also can't pump water back up a river. You need a source of constantly available water (either lake or river with constant flow) and you need somewhere separate to pump it to. I agree it is not totally logical from a location point of view, but it ticks nearly every other box from geography (large basin with steep sides) and also in the middle of nowhere so lower NIMBY component. Still some significant environmental issues that would need to be considered though. Totally agree there should be more money spent on energy efficiency initiatives, that is supposedly the best bang for buck. But longer term Onslow is a reasonable proposition if the pricing works out at the current numbers being bandied about.

I think it would be possible, not by raising the level and flooding Taupo, but by lowering the level capability of the existing outflow and channel.
I suspect this could be done, without impacting on Huka falls, or the first dam on the Waikato.
This would have the major advantage of having supply close to demand, and not need an expensive transmission rejig, and vulnerability to outage and line loss (resistance) of cable and lines all the way from the South Island.
Be Much cheaper than Onslow!
Something for the engineers, cost accountants, politicians to seriously consider before committing major capital to Onslow...