Gina Rinehart's eldest daughter has given evidence against her mining magnate mother in a new court feud over the family's finances.

Bianca Rinehart and her brother, John Hancock, lodged a Federal Court case to try to increase their stake in Hancock Prospecting.

Gina Rinehart controls just more than 76 per cent of the company. Her children control just more than 23 per cent.

The siblings are also seeking access to records of mining profits, accusing their mother of defrauding them of billions of dollars.

The pair are disputing the ownership of shares and assets in their mother's mining companies.

Under cross-examination by Gina Rinehart's lawyer in the Federal Court in Sydney, Bianca Rinehart was asked if she printed confidential emails and kept them in a folder at work when she was a director of Hancock Prospecting several years ago.

Bianca Rinehart said they were important emails and she needed to keep them, but she denied keeping them in the office.

"That would be quite stupid of me to do," she said.

She said she kept the emails at home and that, at one point, her work emails had been "compromised".

She denied that some of the emails contained company information that she had accused her mother of deliberately withholding from her.

When asked if she was aware the allegation of deceptive conduct she made against her mother for allegedly withholding information was a very serious one, Bianca Rinehart quickly replied: "Yes, and I stand by it".

Lawyers for Gina Rinehart previously argued the case should be dealt with privately because it might do commercial damage to projects such as the multi-billion-dollar Roy Hill mining project in Western Australia.

The court case is separate to a long-running dispute over who should control the Hope Margaret Hancock Trust set up by Gina Rinehart's father Lang Hancock.

That case remains unresolved.