Gina Rinehart has to release the tax advice she relied on to try to force her children to sign away their rights to a $5 billion family trust.
The advice from accounting firm PricewaterhouseCoopers was pivotal in triggering the bitter legal battle with three of her children which erupted in September.
It was related to whether the children would be liable for capital gains tax if they claimed their $5 billion mining inheritance. Until now, Mrs Rinehart has claimed PwC would not allow its release.
However, in a judgment late on Wednesday, NSW Supreme Court Justice Paul Brereton ruled Mrs Rinehart must provide a copy.
The trust was due to vest on September 6 when Mrs Rinehart's youngest child Ginia, who is siding with her mother, turned 25.
But three days earlier, Mrs Rinehart told Hancock Prospecting chief financial officer Jay Newby to email her other children, seeking more control of the trust. She said vesting it would trigger a huge capital gains tax bill that would "ruin" them.
When told of their mother's decision, the three older children, Bianca Rinehart, Hope Welker and John Hancock, were overseas. They refused to sign and requested immediate access to all the trust's accounts.
Unbeknown to them, Mrs Rinehart had varied the trust vesting date to 2068. She has since reversed that and the trust has now vested, though the children have yet to get any money because Mrs Rinehart controls the distribution of related company funds.
The children are seeking in court to have their mother removed as trustee.
Mr Hancock said yesterday he welcomed the decision but said it had taken too long and should not have taken a court order.
"We now believe there was never a risk we could become bankrupt and have sought advice from the Australian Tax Office to confirm the advice of our senior counsel," he said.
"When we finally get to see what was in the mystery PwC advice, we can hopefully piece together exactly why our mother wrote that letter that put so much pressure on us with the threat of financial doom."
Mrs Rinehart did not respond to questions.