A total of 152 koalas were put down after contracting a disease from a nearby American-owned aluminum smelter.

The company recently admitted to killing the marsupials after learning the creatures were deteriorating from exposure to fluoride - a waste product of smelting.

The industry giant found the large group of marsupials living in the 17 hectares closest to the smelter were suffering from the effects of fluorosis.

The 152 worst-affected koalas - about 60 per cent of those assessed were euthanised and the forest was closed off from the public in March.