Former paper mill in Mataura, Southland.
Photo: RNZ / Rebekah Parsons-King
The waste - a by-product of aluminium production at the Tiwai Point smelter - releases deadly ammonia gas if it gets wet.
The Southland town's old paper mill site is situated next to the Mataura River.
The dross is a form of metal-rich scum off the top of aluminium processing pods, that is processed to remove the metal into a product that can then be converted into other materials, including ouvea premix which itself is a hazardous substance.
Environment Minister David Parker said the last bags of dross were moved from Mataura last Wednesday.