With access to the trust, the children could press their rights as minority shareholders for representation on the board, leaving Hancock Prospecting more open to external influence, legal experts say, just as Rinehart is trying to wrap up funding for a $10 billion iron ore project.
"They will become substantial shareholders and may push for influence in management decisions, rather than being passive shareholders," said Professor Michael Adams, dean of the law school at the University of Western Sydney.
John, Bianca and their sister Hope Rinehart Welker, 28, sued in 2011 to remove their mother as the trust manager after she pushed out its vesting date until 2068, meaning the children would not get their shares until they were in their 80s and 90s.
Rinehart warned they would be hit with a huge tax bill if the trust was allowed to vest and offered them each $300 million to accept the change in the vesting date.
Lawyers for the children are also examining changes made by Rinehart to the rules governing the trust, barring the children from transferring their shares to each other and giving Rinehart preemptive rights to buy the shares if any of them want out.
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Rinehart later made concessions -- moving the vesting date to April 2012 and, ahead of the trial, stepping down as trustee.
She said the changes to the company constitution overseeing the trust were necessary because she needed total control for negotiations with Rio Tinto to form a joint venture to develop the Hope Downs mine.
Lawyer Christopher Withers, representing John and Bianca, told the New South Wales Supreme Court there was no evidence to support this and argued the changes were for Rinehart's benefit.
Rinehart's hard head for business is legendary. The so-called "Pilbara Princess", the only child of Australia's most prominent frontier miner, has used a string of lawsuits to protect her riches, estimated by Forbes at $18 billion.
Rinehart, 59, has multiplied the fortune left by her father, Lang Hancock, who is credited with discovering the world's largest deposit of iron ore in Australia's Pilbara region.
"The value of the trust property has grown enormously through Mrs Rinehart's responsible endeavors and very hard work," her lawyer Paul McCann said.
Among the revelations in court was an email from Hope, who pulled out of the case earlier this year, to her mother complaining about being "down to my last $60,000". Younger sister Ginia, 27, who has sided with her mother in the dispute, told Hope in another email to "just take the 300 million dollars mum has repeatedly offered you and walk away."
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Rinehart, in turn, has described her elder children as lazy, spoilt and lacking skills to run the multi-billion-dollar trust.
Just who will become the new trustee -- and have responsibility for managing a shareholding controlling almost a quarter of Hancock Prospecting -- will now be determined by the court after the family was unable to agree on a candidate.
Judge Paul Brereton has asked for further submissions before making a decision, while there will be further hearings to decide whether anyone is liable for damages.
While the children's combined shareholding is just shy of the 25 percent needed to pass special resolutions, it puts them in a position to press their rights as minority shareholders and seek more power via the courts, legal experts say.