The heirs of Lang Hancock's former business partner, Peter Wright, are fighting in the West Australian Supreme Court for multibillion-dollar stakes in a series of Hope Downs assets he and Lang Hancock discovered in the 1950s.
Their lawyer Julie Taylor on Tuesday said dozens of letters penned by Mr Hancock in the decades after showed he worked on behalf of the men's partnership agreement to secure the tenements for the areas and open them up for mining.
One written to Mrs Rinehart on February 20 in 1986 discusses Mr Hancock's attempts to divide up some of Hancock Prospecting's assets between himself and his daughter.
It details a series of tenements, including those held jointly by the Hancock and Wright business partnership.
"This is very significant evidence that both Lang and also Mrs Rinehart knew and understood that all of those areas were held jointly for the partnership," Mrs Taylor said.
"(Hancock Prospecting) was only entitled at most to a half (share).
"Mrs Rinehart has known at least since February 1986 that the position with these assets is that they are held jointly with the partnership."
Mrs Taylor told Justice Jennifer Smith the letter came from discovery documents provided by Mrs Rinehart's two eldest children, John Hancock and Bianca Rinehart, not Hancock Prospecting.
The pair are also involved in the complex case, claiming they are entitled to a hefty share in the Hope Downs operation as assets their grandfather left them.
Deceased prospector Don Rhodes' family company DFD Rhodes is also represented in court and claims a 1.25 per cent royalty share of Hope Downs' production over a deal with Mr Hancock and Mr Wright that saw it hand over tenements in the 1960s.
About two dozen lawyers have packed the Perth court for the trial that started on Monday but neither Mrs Rinehart, her children nor Mr Wright's surviving heirs have been spotted.
The case is expected to run until November, with any judgment potentially superseded by the looming outcome of Federal Court arbitration between Hancock Prospecting and Mrs Rinehart's children concerning the same issues.
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 Mr Hancock and Mr Wright discovered at Hope Downs, signing a deal in 2005 with Rio Tinto, which has a 50 per cent stake in the project.
Her wealth is estimated to be about $36 billion.
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.
