The Environmental Defence Society has filed Environment Court proceedings against New Zealand Aluminium Smelters over the 10,000 tonnes of so-called ouvea premix dumped in the small Southland town in 2014.
The waste, which releases toxic ammonia gas when wet, caused concerns in February when the swollen Mataura River swept through its namesake's streets during flooding around the Gore District.
The ouvea premix is a by-product of aluminium production at Tiwai Point.
While New Zealand Aluminium Smelters (NZAS) contracted Taha Asia Pacific to take care of the waste product in 2011, the Environmental Defence Society was arguing that contract - and Taha's subsequent liquidation - should not absolve the smelter of responsibility.
The society's chief executive, Gary Taylor, said they were asking the court to decide who had responsibility under the Resource Management Act.
"Our contention is that the aluminium smelter has that responsibility, notwithstanding any contractual arrangements they've made to pass it on to other people," Taylor said.
The society sought court-assisted mediation as a first step.
"I would expect that could happen pretty promptly," Taylor said.
"If there was a need for a hearing, the Environment Court is pretty efficient these days, I would expect that we'd get this matter resolved by the end of the year."
In March 2018, local authorities, the Government and the smelter agreed to a $4 million plan to move the waste from Mataura and other sites around Southland over six years.
Taylor said the legal action did not change that.
"We're saying that as long as that material sits there and is a potential risk to human health then that's their responsibility and they need to acknowledge that.
"If they did that I think you'd find the material was moved much more quickly to a safer site."
The Environmental Defence Society's approach had the support of Gore District Council.
Council chief executive Steve Parry thought a handshake deal had been reached to expedite the removal of the waste following February's floods, but the smelter backed out after its principal owner Rio Tinto got involved.
Mayor Tracy Hicks said the council had provided the Environmental Defence Society with information to assist its claim.
"If there has to be blame laid at the foot of this particular issue, we believe - along with the Environmental Defence Society - the blame lies with Rio Tinto, who are the owners and manufacturers of the product," he said.
"At the moment we have a situation where we have got a toxic product stored not just in Mataura, but across the province, that the community - be it the local community and the national community - are having to pay to clean up."
Sort Out The Dross spokesperson Cherie Chapman said she also supported the court action and hoped New Zealand Aluminium Smelters would be forced to clean up the waste.
"NZAS and Rio Tinto have to take ownership of this stuff," she said.
"It's gone on too long and it's been sitting around not only Mataura but Southland for a very, very long time. So I think it's a really good idea to establish who owns this stuff."
Environment Minister David Parker declined to comment on the court action, but said negotiations with the smelter were ongoing.
More than 1000 tonnes of the ouvea premix had been moved from Mataura so far under the 2018 deal, but that was not fast enough for the community.
