Alcoa's Australian assets have faced higher energy costs since the carbon tax was repealed, in revelations that challenge one of the primary criticisms of the controversial scheme.

The higher energy costs were revealed on Thursday by Alcoa's local joint venture partner, Alumina Limited, which said its energy costs had risen in the six months since the carbon tax was repealed, once currency fluctuations were put aside.

A giant kettle pours molten aluminum into moulds at an Alumina smelting plant. Credit:Bloomberg

"Excluding the currency impact, energy costs were higher than 2013 levels mainly due to increased prices and the loss of carbon tax credits in Australia," the company said.

The outcome demonstrates the extraordinary level of compensation granted to big industrial companies under the carbon tax, in a bid to shield them from rising power prices.