Commissioner of Taxation of the Commonwealth of Australia v Mount Isa Mines Limited

The Commissioner of Taxation of the Commonwealth of Australia applied for special leave to appeal to the High Court of Australia against decisions of the Full Court of the Federal Court of Australia concerning deductions claimed by Mount Isa Mines Limited. The dispute involved two separate claims for deductions: one relating to the wall of a tailings dam at Mount Isa, and the other concerning expenses incurred in the construction of a decline at Agnew in Western Australia.

The primary legal issue before the High Court was whether expenditure on the tailings dam wall constituted an allowable deduction under Division 10 of the relevant tax legislation. Specifically, the court had to determine if the wall qualified as "plant" necessary for carrying on "prescribed mining operations," as defined by the Act. The definition of "prescribed mining operations" was central to the dispute, referring to the extraction of minerals from their natural site.

The Full Court of the Federal Court had allowed a deduction for the tailings dam wall, but the reasoning differed among the judges. The primary judge had found the expenditure to be on plant necessary for prescribed mining operations. However, all members of the Full Court rejected this reasoning, concluding that the tailings dam wall was not plant necessary for the actual extraction of minerals from the ground. Instead, it was considered an adjunct to the treatment process that followed extraction, used to contain waste materials and noxious substances from the treatment plant.