Former attorney-general Christian Porter has revealed a blind trust paid some of the legal fees in his defamation case – and that he doesn’t know where the money came from.

Mr Porter, who is now Industry Minister, was the country’s top legal officer when he launched his legal action against the ABC and journalist Louise Milligan earlier this year.

The revelation about his fees in the case came after he updated his register of interest to Parliament on Tuesday. He said a contribution came from a blind trust known as the “Legal Services Trust”.

“As a potential beneficiary I have no access to information about the conduct and funding of the trust,” he wrote.

Mr Porter sued the ABC and Milligan for defamation earlier this year over a Four Corners story that referred to an unnamed cabinet minister accused of an alleged historical sexual assault. He eventually revealed himself as the accused man, but has strenuously and repeatedly denied any wrongdoing.

In May, the case was settled with no payout.

Mr Porter’s register of interests was updated on Tuesday.

In June, ABC managing director David Anderson told a Senate estimates hearing the broadcaster had agreed to pay $100,000 to Mr Porter’s lawyer Rebekah Giles and half the mediation costs. Including that, he said he thought the case had so far cost the ABC about $780,000.

Mr Porter’s register of interests statement also mentioned his barrister Sue Chrysanthou had not charged for every hour she had worked on the case “consistent with her practice for individual clients”.

It’s also consistent with Ms Chrysanthou’s statement in a related case before the Federal Court that she worked but did not charge for dozens of hours “having regard to the fact my client is an individual and that the number of hours spent on the matter is already significant”.

“Although all of the above contributions were made to me, or were for my benefit, in a purely personal capacity, in the interest of transparency and out of an abundance of caution I make this disclosure,” Mr Porter said.

“Part contribution to the payment of my fees by a blind trust known as the Legal Services Trust. As a potential beneficiary I have no access to information about the conduct and funding of the trust.”

• See the full update here

Ms Chrysanthou and Ms Giles declined to comment when approached on Tuesday.

Labor has demanded answers on who was behind the blind trust.

“Christian Porter’s claim to have no idea who funded his million-dollar legal case is an outrageous abuse of his office,” shadow attorney-general Mark Dreyfus QC said.

“The Australian people need to know who set this trust up, who funded it, how much they donated, and whether they expected to get anything in return for these donations.

“If Mr Porter genuinely doesn’t know who his donors are he shouldn’t accept their money.”

A spokesman for Mr Porter said his disclosure was “in accordance with the requirements of the register and consistent with previous members’ disclosure of circumstances where the costs of personal legal matters have been mitigated by contributions or reductions in fees”.

“No taxpayers’ funds were used in meeting the costs of the minister’s actions against the ABC and Milligan, which have now concluded,” the spokesman said.

In May, Mr Porter was asked about who might be paying his legal fees in the case – estimated to run into the hundreds of thousands of dollars. He repeatedly refused to guarantee funding it all himself, and also would not rule out allowing a supporter to pick up the tab.

He also said repeatedly that he would disclose to Parliament anything he was required to.

Prime Minister Scott Morrison also ruled out taxpayers footing Mr Porter’s legal bill.

-with AAP