Tiwai Aluminium Smelter operators have revealed there are tens of thousand more tonnes of highly toxic waste stored at its Tiwai Point than it has previously reported.
The Rio Tinto-owned company says an independent audit has revealed there is 217,000 tonnes of the spent cell liner (SCL) waste.
In March it said there was 181,000 tonnes of material, and an additional 58,000 tonnes had been sold and sent overseas. The new total arose from annual financial reporting.
New Zealand Aluminium Smelter (NZAS) chief executive Stew Hamilton said the material was stored safely and the additional waste will not slow the timeline for its removal.
SCL contains hazardous substances including fluoride and cyanide, and can result in explosive gases when mixed with water.
RNZ queried the reported number in March, as production figures suggested at least a further 20,000 tonnes of the waste was unaccounted for.
NZAS plans to close the smelter at the end of 2024, and environmental experts are currently checking through 400 samples from 200 sites related to the smelter.
Experts have warned sea-level rise could encroach on waste in storage at the site within a century, which would create a dangerous situation. However, the company had disputed those findings.
In March, the government suspended talks with the company until it provided more details on how it would back up promises to clean up the SCL.
Environment Minister David Parker described them as "uncooperative", and said they had not been open about the state of the site.
An Environment Southland report included information from former staff who claimed highly toxic materials had been buried in unmapped spots at the smelter. And the council report concluded it was "likely that a number of unmapped or unconsented contaminated sites exist".
In a media release Hamilton said SCL stored at the site is "securely and safely stored."
"I want to be very clear that regardless of the amount, we remain committed to removing all of the material from the site to be recycled or disposed of safely and securely in accordance with modern standards. This adjustment will not have a material impact on the timeline for removal."
The company has previously said it was trying to find a buyer for the SCL.
Rio Tinto and NZAS also face controversy around 7000 tonnes of toxic ouvea mix stored at a disused papermill next to a river in Mataura, which it has agreed to remove.