Mr Joyce did not respond to repeated requests for comment on his highly personal email to one of Mrs Rinehart's children to drop her legal action.

The Opposition demanded that Prime Minister Tony Abbott haul in his Agriculture Minister and demand an explanation for the "extraordinary" intervention.

John Hancock, the estranged son of Ms Rinehart, Australia's richest person this week won an epic legal battle over control of the family trust, with his sister and ally Bianca Rinehart appointed trustee.

NSW Supreme Court Justice Paul Brereton appointed Bianca, 38, trustee of the Hope Margaret Hancock Trust, which was set up by her late grandfather Lang Hancock.

In handing down his judgment, Justice Brereton said he had "never seen such pressure exerted, so persistently, on a litigant", as had occurred in the case.

Some measures "closely approached intimidation", Justice Brereton said.

The judgment cited two letters sent by Mr Joyce and the now retired Liberal MP Alby Schultz to Mrs Rinehart's other daughter, Hope Welker.

In the email Mr Joyce noted he has "never met you" but stressed that "Australia needs strong families and Hope we need, really need, yours to be strong".

"Your family are good people, if they were not I would not care as much about you and your mum," the email continued.

"You are a family Australia needs.  All good families have their problems but before it gets really out of hand, I would try to get it back in house and out of public view."

The note was sent from Mr Joyce's parliamentary email address and describes in detail how his own family had been wracked with conflict over his decision to marry his wife.

"I was the one who had been flicked because I had the temerity to choose my own wife away from the New England stud book," he said.

He suggests lawyers would not have Hope Welker's  interests at heart.

"Papers and lawyers get you to say nasty things. They stir you up then turn the account clock on and watch the money flow," he said.

"Who has a motivation and a business based on stirring people up, telling people they can win things, telling people they can get more money and billing for their grief. Hope believe me, such a practise with laywers has been going on a long time, it is their very business."

Senator Joyce also criticised the media.

"Hope, I live in the public gaze, a lot of my life I do not enjoy because every move is on television and in the press. But Australia asks different things of different people.

Hope, once the public interest starts looking into your private life then every mobile phone is one step away from you being on You Tube. You can not control it."

Mr Hancock said on Friday that Mr Joyce had questions to answer and raised doubt over whether he was should continue serving as a minister.

"Is this the type of person who should be sitting in federal cabinet?" Mr Hancock said. .

Mr Hancock said that, by using his government email address to send an email urging Mrs Welker to drop the court action, Mr Joyce had involved the Abbott government in the dispute.

"This character sits three chairs down from the Prime Minister. To have someone so easily manipulated, or alternatively so naive, is dangerous in such a position," he said.

"By doing so, he endorsed the conduct of Gina Rinehart and involved the Australian government."

Mr Joyce was the Coalition's regional development, local government and water spokesman when he emailed Mrs Welker on September 11, 2011.

 Mr Joyce and Mrs Rinehart have a long-standing friendship. The businesswoman was present for Mr Joyce's maiden speech in the House of Representatives after the 2013 election and she flew him as a guest to a lavish wedding ceremony in India in 2011.

"The question then is, what other actions has this puppy dog made either due to outside manipulation or gross naivety?"

Joel Fitzgibbon said Mr Joyce had questions to answer.

"His intervention is as extraordinary as his relationship with Gina Rinehart which requires further explanation," Mr Fitzgibbon said.

"It is very clear he was acting in the interests of Gina Rinehart. If the Prime Minister hasn't hauled him in already he must and secure from him an explanation and test whether anything he has done was unlawful."

Speaking from his hospital bed, Mr Schultz said he and his wife had emailed Hope from their private account in a personal capacity as family friends and launched a passionate defence of his long time friend Mrs Rinehart.

"I know what a lovely kind person she is and I've never seen the person talked about by her children," Mr Schultz said  on Friday.

"I'm pretty angry with the Australian press because she is not like what she is portrayed to be."

Mrs Rinehart's Hancock Coal donated $22,000 to the Liberal Party and $5000 to the ALP in 2011-12.

In 2012-13, Hancock Coal gave $55,000 to Labor and $45,000 to the Nationals. Hancock Prospecting gave $55,000 to the Nationals and $8250 to Queensland's LNP.

An executive at Hancock Prospecting has previously said the donations had nothing to do with Mrs Rinehart directly.