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Gina Rinehart has struggled to hold on to her spot in Bloomberg's register of the world's 40 wealthiest people after becoming the only one of the listed billionaires to suffer a drop in fortune last year.

Her diminished fortune, which according to Bloomberg fell by $US1.6 billion ($1.5 billion) in the year to December 31, resulted in her ranking 39th and saw Mrs Rinehart swap positions with the now-wealthier Indian steel tycoon, Lakshmi Mittal.

Bloomberg did not spell out why it had cut Mrs Rinehart's stake, although it is likely the iron ore price weakness as well as her struggle to secure finance for the $10 billion Roy Hill project played a role.

Bloomberg provides a daily update of what the news service calculates to be the world's 100 wealthiest people. Italian industrial tycoon Paolo Rocca, with a worth of $US10.4 billion, was ranked 100th at December 31.

Mrs Rinehart's fortune is based on profit shares and royalties from Pilbara iron ore mines such as Hope Downs.

Her ascension to the list of the world's super rich occurred last January when Posco agreed to invest in her Roy Hill venture. Posco's investment placed a $10 billion value on the yet-to-be developed Roy Hill, in which Mrs Rinehart retains a 70 per cent stake, and resulted in the publicity-shy Perth billionaire attracting attention from the world's rich lists.

Just as Mrs Rinehart's fortune suffered a downturn last year when the price of iron ore, shipped to China, tumbled to $US86.70 a tonne in September, her purported wealth appears on the ascent again. By December 31, when Bloomberg last updated its billionaires index, the iron ore price had soared to $US144.90/t.