The costs were immense, the risks high and the environmental impact considerable.
National, Act and New Zealand First have all said it has no future.
However, the "dry year" problem will not go away. When the South Island hydro lakes are starved of water, as happens about every seven years, the country’s electricity capacity will be dire.
New Zealand at present relies on burning coal and gas to cover gaps. That contingency is likely to need to continue, despite the 2030 goal of 100% renewables.
In any case, Onslow would not have been completed for at least another decade.
Onslow is, in principle, a sound idea, and pumped hydro storage is used successfully in some parts of the world.
Water would be pumped from Lake Roxburgh to a vastly expanded Lake Onslow at times of the day and year when demand was low and hydroelectricity super cheap.
It would be released back to the Clutha River to generate power when demand was at its highest and saved for that non-rainy day in the hydro catchments.
Onslow was to be so big that it would have stored the equivalent power of 12% to 20% of annual electricity use. The potential generation at any one time would have been more than twice the Clyde Dam’s.
Early estimates of the costs were about $4 billion. The figure is now closer to $16 billion. Who knows what it could have risen to?
About $20 million has so far been spent on investigations. About another $70 million had been earmarked.
Phase 1 concluded that extending the lake and building the underground power station appeared geotechnically feasible.
Eight recorded archaeological sites would be lost, one with "high" significance and five of medium significance.
Reactions in the Teviot Valley and Roxburgh will be mixed.
The gigantic project would have created many jobs for many years. It would have also created large-scale disruption.
Electricity generating companies will be pleased. Onslow would have curtailed windfall profits.
They reap their biggest return when power is in short supply and expensive.
National’s answer is to make consenting for renewable generation easier. But the only way to compensate for dry years is to hugely "overbuild". About 80% of New Zealand’s power is from hydroelectricity.
Overbuilding is not only a huge cost (including the environmental impact and carbon footprint of constructing so many extra wind turbines and other renewables), but it would again be counter to the interests of the generating companies.
They have sat on wind farm consents over the past decade without proceeding. Why should they overbuild when, under the current electricity market, that would undermine their profits?
MBIE has canvassed other dry-year options. None appeared practical.
The generators speak of "demand management". While this must be a significant part of the equation, it is far from a total answer as electricity demand climbs after several flat years.
If peak use, notably on early winter evenings (also when the sun does not shine) can be reduced, far less capacity has to be built.
There should, for example, be incentives not to charge cars at peak times. Perhaps, households will also use car batteries to supply energy back into the home during that period.
Technological and storage advances could also turbocharge the spread of solar generation.
The battery, in the foreseeable future, has run flat on the Onslow project.
It was the ultimate in think-big solutions and might have dealt with the dry-year electricity woes.
It was, though, putting far too much water into a one-lake option. The risks and costs were too high.
Instead, New Zealand will likely have to muddle through.
