Fortescue Metals Group Ltd (FMG) said on Thursday that lingering reservations on the proposed minerals resource rent tax were holding back the firm's final decisions on two impending giant projects at Solomon and Western hub in Western Australia.

Company executive director Russell Scrimshaw said that in spite of the federal government's decision to replace the controversial resource super profits tax, Fortescue is still wary of the new tax's embodying provisions.

Mr Scrimshaw added that the Fortescue's board "are still sufficiently concerned as to the uncertainty of the final structure of the proposed tax that we have not yet taken any firm decision on the projects that are on hold."

In that line, the company has decided that it is appropriate for now to put on hold the two projects lined up for Western Australia as he gave assurance that "when we do have sufficient clarity around the tax, we will inform the market of our decision at that point."

The government said that the new tax would only affect coal and iron ore projects and basing on the official figures released by the Treasury, the MRRT, which was agreed upon by giant mining companies and the federal government, is poised to collect up to $10.5 billion in revenues from the first two years of its implementation.

Fortescue's latest production results showed that its June quarter output saw a sharp increase on its iron ore mines and shipments as compared to the result registered from the previous corresponding period.