Andrew Forrest's Fortescue Metals Group stands the Gillard government's proposed minerals resources rent tax (MRRT) should not proceed in its current shape.

Fortescue's head of government relations Deidre Willmott told a Senate hearing that they were not consulted on the MRRT.

The revamped resource tax, announced on July 2, was negotiated with three major mining companies, BHP Billiton, Rio Tinto and Xstrata.

"In our view the MRRT should not be implemented," Ms Willmott said.

"It is complex, discriminatory and a $10.5 billion impost on the resources industry."

According to her, the absence of transparency on how that $10.5 billion is arrived at was a major concern to Fortescue and other miners.

Fortescue's chief financial officer Stephen Pearce said the group was "adamant" that further revisions were required to better consider the needs of emerging and developing iron ore miners.

He said the levy needed to reflect the "practical reality" of this sector to achieve a fairer outcome for emerging companies, adding that there were several key items that needed to be changed to give clarity and certainty to the industry, including tax deductability.

Mr Pearce said Fortescue remained against a tax of this scale that imposed a duty prior to interest and other costs being paid.

He also said the negotiated agreement favoured large multinational companies that have access to cheaper funds, while the MRRT did not consider items such as the extraction allowance and infrastructure capital for smaller miners.

"The MRRT threshold should be increased to $100 million to encourage growth for smaller players," he said, rather than the $50 million proposed.