The heirs of Mrs Rinehart's father's former business partner, Peter Wright, are fighting for multibillion-dollar stakes in a series of assets in WA's northwest he and Lang Hancock discovered in the 1950s.
The complex legal stoush also involves Mrs Rinehart's two eldest children John Hancock and Bianca Rinehart who, like their mother's company Hancock Prospecting, are defending themselves against Wright Prospecting's claim.
The pair assert they are entitled to a hefty share in the Pilbara region operation known as Hope Downs, amid a fraud allegation against their mother, which they say defeats the Wrights' claim.
But the Wrights' lawyer Julie Taylor said on Thursday her client's interest in the project, as a co-owner of tenements, pre-dates that of John Hancock and Bianca Rinehart and trumps their claim.
She also said Mrs Rinehart and her mining pioneer father knew about the Wright's involvement and claim when the children were allegedly bequeathed an interest.
"John and Bianca's claim is a distraction that will not affect the outcome of this case," Ms Taylor said.
"Even if the very serious allegations of fraud could be made out without any witnesses it would not sound in the proprietary remedy that would be required in order to defeat (Wright Prospecting's) interests in Hope Downs and the East Angelas."
She said that if Justice Jennifer Smith accepted the Wrights co-owned the tenements in the areas "then there was no fraud involved in the 1992 and 1995 agreements".
The court heard the children claimed they had been left shares in a company that controlled mining assets and Mrs Rinehart had allegedly devalued it in a bid to decrease their control of Hancock Prospecting.
Ms Taylor said the pair's defence relied on a 1988 agreement involving a trust to assert their claim, but Ms Taylor said it had nothing to do with the ownership of the tenements.
"Its object was the making of arrangements between Lang and his daughter, Mrs Rinehart, and her children concerning shareholdings and control of (Hancock Prospecting)," she said.
"None of the matters relied upon by John and Bianca in support of their claimed interests as beneficiaries of the alleged ... trust, even taken at their highest, establish the children have or ever did have proprietary interest in the Hope Downs and the East Angelas."
About two dozen lawyers and company spin doctors have packed a Perth courtroom for the trial since it started on Monday, which also involves the family company of deceased prospector Don Rhodes, DFD Rhodes.
That company claiming a 1.25 per cent royalty share of Hope Downs' production, over an alleged deal with Mr Hancock and Mr Wright that saw it hand over tenements in the 1960s.
Mrs Rinehart inherited her father's iron ore discovery in WA's Pilbara region and forged a mining empire after he died in 1992.
She developed mines from the tenements at Hope Downs, signing a deal in 2005 with Rio Tinto, which has a 50 per cent stake in the project.
Australia's richest person, her wealth is estimated to be about $36 billion and she is executive chair of Hancock Prospecting.
The Hope Downs iron ore mining complex is made up of four open-pit mines near Newman.
It produces more than 45 million tonnes each year and is one of Australia's largest and most successful iron ore projects.
Wright Prospecting has demanded a share of unmined and mined Hope Downs tenements and royalties amid a claim that Hancock Prospecting breached a series of partnership agreements.
The trial, which is expected to run until November, is not hearing Mr Hancock and Ms Rinehart's claim for a share of the Hope Down's project, involving the allegation against their mother.
Hancock Prospecting maintains it undertook all the work, bore the financial risk involved in the development and is the legitimate owner of the assets.
