Decades old private correspondence between Lang Hancock and his daughter Gina Rinehart have shed new light on their bitter relationship breakdown, with the mining pioneer accusing his only child of prompting him to sign a legal document under duress.

Letters between Hancock and Mrs Rinehart, now Australia’s richest person, were aired in the WA Supreme Court on Wednesday during a marathon four-way legal battle over the rights to the family’s mining riches.