The pair were joined on the list by Westpac chief executive Gail Kelly, who slipped from a ranking of eight in 2010 to 32 in this year's edition.
German chancellor Angela Merkel was ranked in the number one spot, beating out US secretary of state Hillary Clinton and the President of Brazil, Dilma Rousseff.
Rinehart, who did not feature on last year's list, has rocketed to prominence as the resource boom has inflated her wealth to more than $10 billion.
Forbes described Rinehart as "wielding her bank account for influence".
"She's investing heavily in a campaign against Australia's environmental reforms, including taxes on extractive industries like mining that have a heavy carbon footprint. In addition, she's attracting controversy for subsidising the work of scientists who deny man-made climate change."
Gillard is described as "struggling to gain support on a controversial carbon tax and to implement national health reforms".
While Rinehart, Gillard and Kelly are ranked with some of the most powerful politicians, regulators and business people in the world, the trio could not beat out stars of stage and screen.
Singer Lady Gaga (ranked 11th), television presenter Oprah Winfrey (ranked 14th) and singer Beyonce Knowles were all judged to be more powerful than our richest person, our most powerful politician and the chief executive of one of country's top banks.
The list is based on three factors: money (measured by revenue for business and media women, GDP for politicians and salary for celebrities), media (traditional and social media reach) and the size of the candidate's "power base".
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