The Tiwai Point aluminium smelter clean up and closure costs have reached $687 million, after earlier making provisions for $352m.
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Though the smelter was likely to not close in December 2024 as planned, the company said it would clean up the site on sensitive coastal land opposite Bluff regardless.
The massive jump in costs was due in part to it promising to get rid of more than 200,000 tonnes of spent cell lining waste, that contains cyanide and toxic fluoride, by 2029.
It also allowed for shifting the huge landfill when the smelter did eventually close.
The government fought the smelter's majority owner Rio Tinto for much of last year to commit to more of a clean up.
"We have committed to remediating the site, whether we remain operating beyond 2024 or not," New Zealand's Aluminium Smelter (NZAS) chief executive and general manager Chris Blenkiron said in a statement.
"This updated provision ensures our community can be confident we are putting the right plans in place.
"In the meantime, work is already underway to remove waste as part of our commitment to continue to improve our environmental performance."
The company returned profits of $140m for last year, after threatening in 2020 to close as soon as August 2021 because the business was not viable.
That sparked top level negotiations with government, rebukes from ministers, and eventually, a sweetheart power deal with Meridian that bought extra time.