And it doesn't pay for any of that damage. Instead, Tiwai receives a massive subsidy of 1.55 million tons of carbon a year, about 900,000 tons more than it actually emits. The "justification" for this over-subsidisation is "compensation" for higher electricity prices caused by the ETS. But Tiwai doesn't actually pay those prices, instead being subsidised $350 million a year by the rest of us, at a cost of ~$200 per household.
If Tiwai wants to stick around, then we should end those subsidies. Meridian doesn't sound eager to continue subsidising its electricity costs (having used the last two years to start building alternative demand in the South Island), and that's good. But we should also eliminate its carbon subsidy. In theory that just means removing "aluminium smelting" from the relevant regulation, but to be certain, Parliament should add a clarifying clause to the appropriate section of the Climate Change Response Act specifying that "Aluminium smelting is not an eligible industrial activity". This one clause would reduce emissions by 1.5 million tons a year, saving us $225 million a year in social cost.
If Tiwai really is as clean as they say they are, they should have no problem paying the full cost of their emissions. And if they're not profitable when they do that, then they were never really profitable at all, and we really are better off without them.