The last bag of ouvea premix removed has been removed from the Mataura's former paper mill.

Some 10,000 tonnes of the Class 6 hazardous substance was stored at the mill. It is a by-product from aluminium dross processing at Tiwai Point aluminium smelter, near Bluff.

But six months ago, following legal action brought by the Environmental Defence Society, efforts began to speed up its removal.

Gore District Mayor Tracy Hicks says: "The presence of this substance has caused all sorts of anxiety and consternation for residents.

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"They can now have peace of mind, and not before time."

When wet, ouvea premix releases toxic ammonia gas.

"How anyone thought it was a good idea to store this product in a building next to a river in a semi-residential area is beyond me," Hicks says.

Taha Asia Pacific started storing ouvea premix at the former Carter Holt Harvey paper mill without resource consent in 2014.

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The company applied for and was granted a consent retrospectively in 2015 and then liquidated in 2016.

Hicks says the council took immediate steps to rectify the situation when it became aware of the significant quantity of ouvea and its risk.

Gore District Chief Executive Stephen Parry says getting agreement around the removal of the substance had been a very complex problem.

After 18 months of negotiations, a deal was secured in July 2019 between New Zealand Aluminium Smelter (NZAS), the Government, Southland's local authorities and the landlords of storage sites in the province.

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"All parties felt they were victims and had been let down by Taha. It had been very taxing trying to get people to step up."

Hicks says he was pleased Rio Tinto, the majority owner of Tiwai, had accepted financial responsibility and expedited removal of the substance from storage sites.

The project cost $6 million, with Rio Tinto picking up the whole bill. The last bag was removed last Wednesday. They are being stored in watertight shipping containers at the Tiwai Point smelter.

Parry also acknowledged the "sea change in attitude around liability" evident after leadership changes within Rio Tinto.

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The company has come under fire for the way hazardous waste is stored at the Tiwai Point smelter, close to an eroding beach, but has recently agreed to meet the costs of rehabilitating the site when the smelter closes.

Parry also applauded the efforts of Australian-based company Inalco, which would be taking the premix for processing.

“It has been fantastic to deal with.”

As well as the 10,000 tonnes at Mataura, there were about 12,000 tonnes of ouvea in storage around Invercargill.

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Environment Minister David Parker welcomed the news.

"The safe removal means Mataura residents no longer need to worry about the risk from the material, which gives off ammonia gas if it gets wet.

"I'm glad that the dross has now gone from Mataura and local residents don't need to worry any longer. The safe removal will protect the wellbeing of Southland's people and its waterways.

"Now that the Mataura site has been cleaned up, the focus will move to the removal of material from other known sites in Southland."

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He thanked the organisations that contributed to the successful remediation.

"It has taken a long time to get to this point and I’d like to thank everyone involved including the Gore District Council, Environment Southland, the Environmental Defence Society, Ngāi Tahu, New Zealand Aluminium Smelter and Inalco Processing Ltd."