The federal government’s proposed national integrity commission may be able to examine who contributed to a legal fund that cost Christian Porter his ministerial job, according to the assistant minister to the attorney-general, Amanda Stoker.

In comments that could alarm some of her colleagues, Stoker has suggested the blind trust established to help Porter pay legal fees in an abandoned defamation case against the ABC could fall within the proposed focus of a national anti-corruption body.

“The commission that is currently in the exposure draft design would, I believe, enable an investigation into the source of those funds,” the conservative Queensland senator told ABC Radio in Brisbane.

Released a year ago, the draft bill to establish the commission significantly limits the scope for receiving referrals involving parliamentarians. The proposal relates only to instances where there is a reasonable suspicion of corrupt conduct involving specific listed offences – although The Saturday Paper is not suggesting Porter has committed any offence.

Porter quit the ministry last month after being unable to say who had paid his legal bills. He has recently renominated unopposed to recontest his marginal seat of Pearce for the Liberal Party at the coming federal election.

Stoker was also asked if the proposed commission could examine the past personal investment in a controversial water buyback scheme by the minister who has taken over Porter’s Industry portfolio, Angus Taylor.

“Look, if there was any suggestion that it amounted to corrupt conduct, absolutely,” Stoker said, adding that previous parliamentary attacks on Taylor had been “an extremely scurrilous attempt” to link him to the actions of a “far-flung” family member.

Stoker also advocated the proposed Commonwealth Integrity Commission hold public as well as private hearings. The current proposal divides the commission into two divisions. One, covering law-enforcement agencies, could hold public hearings. The other, covering other public-sector institutions, would not.

“There’s a few different things that crop up in terms of design that are really important,” Stoker volunteered in the radio interview. “One is, ‘Should there be public or private hearings?’ And the sensible answer is probably something that involves a bit of both, right? Depending on the circumstances.”

Her comments appear at odds with the views of Prime Minister Scott Morrison and other colleagues, especially in New South Wales, who do not want the commission to examine events that occurred before it was established.

There are also strong views within the wider Coalition on other aspects of the design.

Victorian Liberal MP Russell Broadbent is advocating against public hearings but warning that the current proposed framework offers politicians too much protection from scrutiny.

“There is so much distrust of government that we cannot afford to get this wrong,” Broadbent tells The Saturday Paper. “For a start, strengthen the conflict-of-interest and criminality provisions in the bill. For instance, the same scrutiny should apply to politicians as to the Federal Police.”

Broadbent says: “The public interest must be paramount here. Politicians must be prepared to face scrutiny for their actions, just like anybody else.”

But he shares the concerns of those critical of the reputational damage that can be inflicted by a process that does not have to meet a threshold test of possible criminality before airing matters – including about witnesses – in public.

“I’m concerned about public hearings,” Broadbent says. But under the proposed structure, “if there’s any criminality found, they would go straight into the public arena”.

Many Liberals are angry that the processes of the NSW Independent Commission Against Corruption sparked the resignation of then premier Gladys Berejiklian last week. Some Labor MPs share their concern. Retiring NSW Labor MP Joel Fitzgibbon called ICAC a “kangaroo court”.

Berejiklian now faces weeks of public hearings as ICAC investigates whether a secret five-year relationship she had with former state Liberal MP Daryl Maguire compromised her role as premier and whether she acted corruptly in allocating grants to his electorate. Berejiklian has denied any wrongdoing, insisting she always acted in the interests of NSW.

The relationship was revealed a year ago when ICAC was investigating Maguire’s business activities.

ICAC phone taps targeting Maguire captured conversations between him and Berejiklian. Some recordings were played in an open hearing when Berejiklian was called as a witness in October last year.

One, from 2017, revealed Maguire telling her about his attempts to broker a $330 million Western Sydney land deal and Berejiklian replying: “I don’t need to know about that bit.”

Under the ICAC Act, ministers must notify the commission of any possible corrupt conduct.

The events in NSW have reignited debate over the powers of a national integrity commission, first proposed three years ago. The government has promised legislation will go before parliament by the end of this year.

Liberal sources emphasised this week that it was crucial to get the model right from the outset, as it could be politically extremely difficult, if not impossible, for any future government to amend it.

Morrison condemned the NSW model as one that could trash the reputations of innocent people.

“It’s certainly not a model that we’d ever consider at a federal level,” he told the Seven Network. “… And you know, you’ve got to have processes that assume people are innocent before thought to be guilty. And that is a real problem … I’m sure there are millions of people who’ve seen what’s happened to Gladys Berejiklian. They’ll understand that’s a pretty good call not to follow that model.”

Coming as the state battles the Delta strain of Covid-19, Berejiklian’s decision to resign rather than stand aside – which she had said ministers should do if they faced allegations – prompted a public outcry in her defence and criticism of ICAC’s processes and timing.

Every Australian state and territory has an anti-corruption commission, although with differing powers and reach.

This week, the Centre for Public Integrity compared the proposed national model with those others and alternatives proposed by independent MP Helen Haines and the Australian Greens.

The centre declared the Coalition model “the weakest watchdog”.

Board member and former judge Stephen Charles, QC, said it fell short on almost every measure. “It would hide corruption, not expose it,” he says. “The inability to hold public hearings and table reports would mean the public is left in the dark.”

Labor is pushing for retrospectivity and public hearings. Amanda Stoker said the Coalition would ensure people were properly held to account but without “the star chamber kind of behaviours” of the “almost rogue” NSW ICAC.

She described it as “a monster” with “broad, sweeping powers of inquisition and compulsion”. She said it had seen lives destroyed over “trivialities”.

Asked on ABC Radio National if she supported a new commission being able to look at past events, the assistant minister said it opened “a whole bunch of other questions”.

“How long? How far? Into what?” she asked. “… Retrospective powers are something that should only be done very sparingly and in circumstances where the people involved should expect for that to occur … Anyone in these kinds of jobs would expect to face that kind of scrutiny already and so the case isn’t too bad for doing that.”

She did not describe the people to whom she referred. Insisting retrospectivity was a matter for cabinet, she added: “I think, at the moment, I would want to allow for all matters to be on the table.”

The debate on the risks and benefits of public versus private hearings came as the ACT Court of Appeal emphasised the importance of public disclosure as part of a criminal trial related to national security.

Lawyer Bernard Collaery is being prosecuted for helping his client, a former spy known as Witness K, pass information to journalists about Australia bugging East Timorese government buildings during bilateral oil and gas negotiations.

The court rejected the federal government’s argument that the entire prosecution should be held in secret. It declared that although holding some hearings in public risked prejudicing national security, no significant risk was likely to materialise.

“On the other hand, there was a very real risk of damage to public confidence in the administration of justice if the evidence could not be publicly disclosed,” a summary of the court’s finding says. “The court emphasised that the open hearing of criminal trials was important because it deterred political prosecutions, allowed the public to scrutinise the actions of prosecutors, and permitted the public to properly assess the conduct of the accused person.”

The matters before anti-corruption commissions are governed by less strict rules than criminal proceedings, with fewer legal protections for either those facing allegations or witnesses. Allegations must also meet a higher test before they go before a criminal court.

ICAC cannot prosecute people but can recommend that the director of public prosecutions consider criminal charges. Two former state Labor ministers, Eddie Obeid and Ian Macdonald, have been prosecuted as a result of ICAC investigations.

Gladys Berejiklian is the third NSW Liberal premier after Nick Greiner and Barry O’Farrell to resign in relation to ICAC investigations. Neither of her predecessors faced any criminal charges.

O’Farrell resigned after denying – incorrectly – that he had received an undeclared $3000 bottle of Grange Hermitage wine as a gift. He is now Australia’s high commissioner to India.

Greiner was found to have been “technically corrupt” by encouraging political rival Terry Metherell to leave politics by offering a lucrative public service job. The finding was later overturned by the Court of Appeal.

As the creator of ICAC, Greiner felt very acutely the political consequences of establishing such a body. It remains to be seen whether the federal government will expose the former attorney-general who was originally tasked with designing the national version, Christian Porter, to the same experience.

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