Gina Rinehart's son slams Barnaby Joyce

Federal cabinet minister Barnaby Joyce has been slammed for writing to one of Gina Rinehart's daughters, urging her to abandon legal action against her mother.

The mining magnate's estranged son, John Hancock, on Friday claimed that Mr Joyce emailed his younger sister, Hope Welker, from his government account during a long-running battle over a family trust, estimated to be worth up to $5 billion.

A NSW Supreme Court judge on Thursday named eldest daughter Bianca Rinehart as the best person to be trustee, replacing her mother.

Welcoming the decision, Mr Hancock criticised Mr Joyce for becoming involved in the dispute, a move he described as "extraordinary".

"I think it's nothing short of dangerous," he told ABC radio.

"This character sits three chairs down from our prime minister."

Mr Hancock said the matter had taken its toll on him and his sisters over more than three years.

Bianca Rinehart will now go through the trust records and take appropriate action, he said, with other court proceedings still under way.

He expects his mother will try to appeal the decision.