Prime Minister Scott Morrison has sought advice on whether Christian Porter's decision to accept anonymous payments for his legal case against the ABC has breached ministerial standards.
Mr Porter on Monday updated his register of interest, noting a "blind trust known as the Legal Services Trust" had made a "part contribution" to cover the costs of filing the lawsuit against the public broadcaster.
After criticism was aired over a lack of transparency, a spokesperson for the Prime Minister on Wednesday evening said Mr Morrison had raised the matter directly with Mr Porter earlier in the day and he was taking the matter "seriously".
"The Prime Minister is seeking advice from his Department on any implications for the ministerial standards and any actions the Minister must take to ensure that he meets the standards," the spokesperson said.
Mr Porter has defended the disclosure as acting in accordance with the requirements of the register and consistent with the disclosure of personal legal matters.
Prime Minister Scott Morrison. Source: AAP
But former prime minister Malcolm Turnbull - who promoted the former attorney-general to that position - has criticised the declaration.
Mr Turnbull said politicians should not receive money from undisclosed sources.
"This flies in the face of every principle of accountability and transparency in public life," Mr Turnbull told ABC radio on Wednesday.
"I am staggered that Porter thought he could get away with it and I will be even more staggered if the Prime Minister allows this to stand. It is a shocking affront to transparency."
On his register of interests, Mr Porter said he had no access to information about the conduct and funding of the trust.
He sued the ABC for defamation in March over a story that revealed a now-deceased woman's historical rape allegation against a cabinet minister.
The now-Minister for Industry, Science and Technology emphatically denied the allegation and the case was settled before trial.
The ABC paid $100,000 in costs and agreed to put an editor's note alongside the online story, stating the ABC accepted that some readers misinterpreted the article as an accusation of guilt against Mr Porter, and that this was regretted.
Opposition Leader Anthony Albanese has demanded answers on who was behind the blind trust.
"Christian Porter needs to explain today exactly where this money is coming from, and exactly how much it is and what interest there is in people putting forward this money into his trust," he told reporters on Wednesday.
"The idea that he doesn't know ... is quite frankly, just unbelievable and absurd."
A spokesperson for Mr Porter said the minister's 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 spokesperson said.
Mr Turnbull wants the stronger disclosure laws applied to MPs citing that banks and political parties were banned from receiving cash from unknown sources.
"It is so wrong. I'm astonished," he said.
He said disclosure regimes were crucial to stopping the potential for corruption and influence in politics.
"This flings open the door to such extraordinary abrogation of responsibility and accountability," Mr Turnbull said.
Additional reporting by AAP.
