The complex court case pitted Hancock Prospecting, of which Mrs Rinehart is executive chair, against the wealthy heirs of mining pioneer Peter Wright and engineer Don Rhodes amid allegations of decades-old contract breaches.
A 1655-page judgment, handed down in Perth on Wednesday, found Wright Prospecting and DFD Rhodes won some of their claims for spoils from the massive Rio Tinto-operated Hope Downs mining complex, in WA's ore-rich Pilbara region.
Justice Jennifer Smith found Hancock Prospecting would be required to pay royalties, damages, interest and costs, which could be worth hundreds of millions of dollars.
"At the heart of the issues raised by the parties to the proceedings were a number of formal agreements made decades ago between men who were friends or colleagues," she said.
"Who, for some years engaged in harmonious and co-operative arrangements to explore, discover and prospect for iron ore in the East Pilbara."
But Justice Smith dismissed Wright Prospecting's claim for a half share of some of Hancock's iron ore deposits, worth billions of dollars.
Wright had demanded a stake in mined and unmined Hope Downs tenements and royalties amid a claim Hancock breached a 1980s partnership agreement.
DFD Rhodes also claimed a royalty share of Hope Downs' production over an alleged deal with Mrs Rinehart's father Lang Hancock and Mr Wright that handed over tenements in the 1960s.
The bruising encounter also drew in Mrs Rinehart's children, over a previous claim by John Hancock and Bianca Rinehart stating their grandfather left them a hefty share in the Pilbara mining resources he discovered in the 1950s.
Justice Smith said it wasn't necessary to consider the children's defence "as they fail at the first hurdle".
"I hope we can finally put these events from decades ago behind us, and as a united family, celebrate and continue the contribution we have made to Australia," Mr Hancock said in response.
Rio Tinto was also involved in the battle as the joint-venture partner in Hope Downs.
Hancock Prospecting said the mining giant would have to pay some of the royalties following the decision.
The royalty share payable to Wright and DFD Rhodes wasn't a significant issue, amounting to about $18 million annually, executive director Jay Newby said.
But when multiplied by the number of years Hope Downs had been operating, the figure skyrocketed.
Hancock Prospecting rejected the Wright Prospecting and DFD Rhodes' claims during the trial, maintaining that it undertook all the work, bore the financial risk involved in the development and is the legitimate owner of the assets.
Wright said the assets belonged to the enduring partnership.
The result could trigger more costly legal fights, be it appeals against the judgment or to unravel the value of the royalty entitlement.
The 51-day Western Australian Supreme Court trial featured sensational allegations against Mrs Rinehart, which she vehemently denied.
These include that the reclusive billionaire devised an unlawful scheme to defraud her children and allegedly tried to have her father's wife, Rose Porteous, deported in the 1980s amid fears she would inherit shares in Hancock Prospecting.
"Lang Hancock wrote angrily to Gina Rinehart telling her that he had reached the limit of his patience with her and that he was not going to put up with any further interference in his private life or meddling in any (Hancock Prospecting) affairs," Justice Smith said.
Mrs Rinehart inherited her father's iron ore discovery in the Pilbara region and forged a mining empire after he died in 1992.
She developed mines from tenements at Hope Downs, signing a deal in 2005 with Rio Tinto, which has a 50 per cent stake in the project.
The Hope Downs mining complex near Newman is one of Australia's largest and most successful iron ore projects, with multiple open-pit mines.
Mrs Rinehart's wealth is estimated to be about $40 billion.
The case will return to court to hear submissions on costs and orders later in the month.
