Mr Porter sued the ABC for defamation in March over an online article he alleged portrayed him as the perpetrator of a rape in 1988. He denies the allegation and the case was settled in May.

Geoffrey Watson,SC,a former counsel assisting the NSW Independent Commission Against Corruption and now a director of the Centre for Public Integrity,said the matter exposed a “serious weakness” in federal donation laws.

“His decision to even contemplate accepting this money demonstrates appalling judgment. How does he know the money doesn’t come from a malignant source?,” Mr Watson said.

“His declaration simply says he does not know how the trust is conducted or funded. But he doesn’t say he doesn’t know who is behind it.”

Former prime minister Malcolm Turnbull said the decision by his former attorney general was “a shocking affront to transparency” and he should disclose the donors or “give the money back”.

Transparency International Australia chief executive Serena Lillywhite said it was “staggering” that Mr Porter had accepted the money without checking the credentials of the donors.

“The fact that a trust has been used as the mechanism to give this money suggests that opaqueness and secrecy was considered important and was a very deliberate approach. That really raises questions as to why that was deemed necessary,” she said.

University of Sydney professor Matthew Conaglen,an expert in equity and trust law,said it was conceivable that Mr Porter did not know who had set up the trust or who the trustee was,but there was nothing to stop him from asking.

“He could ask,but equally it doesn’t follow that they have to answer that question. That’s where the complicated law comes in. It all depends on the terms of the trust,” Professor Conaglen said.

“It’s possible that it was created by his legal team but it’s equally possible that they wanted to stay away from it,and it was created by somebody else. You can’t tell from the mere concept itself who has set it up.”

Mr Porter’s solicitor Rebekah Giles did not respond to a request for comment.

Mr Turnbull,who fell out with Mr Porter in the final days of his prime ministership,told ABC radio on Wednesday morning that Mr Porter’s declaration was like saying,“My legal fees were paid by a guy in a mask who dropped off a chaff bag full of cash.”

A spokesman for Mr Porter said on Tuesday,“The minister has undertaken disclosure 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.“

Mr Porter said in his declaration that he had declared the arrangement “although these matters have been conducted in a personal capacity and all legal services were engaged in a purely personal capacity,out of an abundance of caution and consistent with approaches adopted by other parliamentarians in relation to the provision of reduced fee or pro bono legal services”.

“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,” he wrote.

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