BHP Billiton's $30 billion worth Olympic Dam copper and uranium mine project faced its strongest opposition on Thursday as the Greens slammed an indenture agreement that stipulates royalty payments by the global mining company to the South Australian government over the mining project expansion will remain fixed for 45 years.

The indenture arrangement, which runs up to 70 years, specifically fixes BHP Billiton's royalties at the existing 5 per cent for uranium and 3.5 per cent for other minerals for the first 45 years. Former Treasurer and Premier Mike Rann signed the agreement with BHP Billiton Chief Marius Kloppers in Melbourne on Wednesday.

However, Greens upper house MP Mark Parnell said the arrangement was too much of an exemption offer for BHP Billiton.

South Australian Deputy Premier Kevin Foley said the 45-year term fixed rate will bring $350 million a year in revenue once the mine is already fully operational.

"All companies want security, they love to know things that will be set in stone forever but...royalties go up and down, mostly they go up," Parnell said in The Australian.

"To lock in a company with its own special act of parliament to not have to pay any higher royalties for 45 years is wrong and I think we have sold ourselves short," he added.

For the agreement to materialise, it must first be ratified by the South Australian government, after which endorsed to both houses of the state's Parliament by Tuesday next week. But there is a threat it will face proceedings delays, most especially in the upper house, where Labor does not have a majority.

If this happens, the Olympic Dam mine expansion might also face postponements in the commencement of construction works. After Wednesday's signing, BHP Biliton is required to issue a formal ''project notice'' within 12 months upon securing approval of state government legislation. The notice indicates the start of construction works on stage one of the Olympic Dam mine.

Mr Foley said the Olympic Dam mine is only an expansion of an existing mine and does not qualify for the South Australian government's royalty regulations for new mines. New mines receive a two per cent discounted royalty rate applicable in the new mine's first five years of operation.

The South Australian deputy premier further said the 45-year indenture agreement was a trade-off with BHP Billiton for accepting to pay those current royalty rates.

Ever since the federal government proposed the creation of a mining tax, state governments have debated heatedly on imposing royalties to miners, and by how much. The Olympic Dam mine, however, is excluded as it is a copper and uranium mine project. The mining tax applies only to coal and iron ore miners.

Located north of Adelaide, the Olympic Dam mine is the world's fourth-largest copper and gold deposit and the largest known uranium deposit. More than 13,000 jobs are expected once works begin and it is expected to contribute to Australia's coffers an estimated $45 billion annually over the next 40 years.

The Olympic Dam mine is seen to yield copper and uranium oxide production by more than quadruple to about 750,000 tonnes and about 19,000 tonnes, respectively, every year.

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