BHP Billiton will suffer little impact during the first five years of the Gillard government's proposed minerals resources rent tax.

This information was confirmed by BHP's chief executive Marius Kloppers.

But mining analysts say BHP and fellow mining giants Rio Tinto and Xstrata will start paying more after several years as generous transitional concessions negotiated with the government begin to expire.

Mr Kloppers estimated last week the group's total effective tax rate in the early years of the tax would be 45 per cent, compared with about 43 per cent under the current regime.

Last year BHP paid $6.3 billion in taxes, including state royalties, on its Australian profit of $14.65bn.

If the minerals tax had been in place last year, and assuming Mr Kloppers' estimate of 45 per cent, BHP would have paid about $6.6bn -- an extra $300 million.

Mr Kloppers' concession that BHP was likely to pay only slightly more tax in the early years of the minerals tax casts further doubt on the government's forecast that the tax would raise $10.5bn in its first two years.

Analysts said most of the minerals tax would initially be paid by BHP, Rio Tinto and Xstrata. Rio and Xstrata have paid less tax than BHP over the past few years.

In axing its resource super-profits tax in favour of the minerals tax, the government cut the headline rate from 40 per cent to 30 per cent.

With a 25 per cent extraction allowance factored in, the effective rate will be 22.5 per cent.

Analysts have suggested the minerals tax's impact on the valuations of BHP and Rio would be relatively minor.

Credit Suisse analysts said yesterday using market value in calculating the minerals tax was the equivalent of "grandfathering" existing assets from it.