The federal government's minerals resource rent tax is unconstitutional because it imposes different tax rates on miners in different states and territories, the High Court has heard.

David Jackson, QC, appearing for Andrew Forrest's Fortescue Metals Group, told the court the tax's uniform application across the states was discriminatory.

"One can't avoid the fact that it is state-based," he said.

The attorneys-general and solicitors-general of Queensland and Western Australia have joined the action, which is being heard before the High Court in Canberra.

They argue that because mining royalty regimes differed among states before the mining tax took effect on July 1, 2012, the effect of the tax was discriminatory.