Environment Minister David Parker has written a forthright letter to the owner of the Tiwai Point aluminium smelter, accusing it of passing the buck rather than dealing with its hazardous waste.
The letter to New Zealand Aluminium Smelters (NZAS) does not, however, explicitly threaten legal proceedings - a course of action Parker had earlier signalled he was considering.
The correspondence was obtained by RNZ under the Official Information Act.
Almost 10,000 tonnes of aluminium dross created by the smelter has been left sitting in a disused Mataura paper mill since Taha Industries, the company commissioned to remove it, went into liquidation in 2016.
The issue won public attention last month after floodwaters crept close to the mill. If wet, the dross could create a dangerous cloud of ammonia gas.
In the letter - dated 27 February - Parker said he was "extremely disappointed" by the response to date from NZAS, which is mostly owned by Rio Tinto.
"I am increasingly dissatisfied at what amounts to a pattern of unsatisfactory arrangements for the disposal of aluminium dross by-product which has meant organisations other than NZAS or its contracting parties have had to pick up responsibility for this material.
"I will continue to explore the government's options in this regard."
Parker said the Mataura site was plainly unsuitable for storage of the waste and urged NZAS chief executive Stewart Hamilton to "urgently address this problem".
The "most appropriate and obvious" solution was to return the dross to Tiwai Point for storage and processing, the letter said.
"I have not been provided with any practical reason as to why Tiwai Point is not a suitable site beyond a need for minor work to secure a weatherproof building."
Parker said the company's "limited commitment" to addressing the stockpile was inconsistent with its espoused "social and community responsibility".
"The wellbeing of Southland's people and its waterways have been put at risk," he said.
In a media release issued on 17 February, NZAS said the waste material belonged to the Crown following Taha's bankruptcy and that the Gore District Council was ultimately responsible for its removal and disposal.
NZAS said it remained committed to a 2018 agreement with the government and local councils to provide $1.75 million to help with plans to safely remove the material.
In his letter, Parker said he contested some of those "assertions" and accused the smelter of taking an approach which "essentially passes the buck to central government and the local community".
NZAS has been approached for comment.