Attorney-General Christian Porter not only has to convince several sceptical cross-benchers to support the government’s idea of an anti-corruption commission, at least one government MP is threatening to cross the floor against the proposed agency unless it gets far stronger inquisitorial powers.

Coalition backbencher Llew O’Brien’s dim view of the proposed Commonwealth Integrity Commission appeared in The Guardian just one day after Porter’s latest attempt to promote the government’s model and counter its many critics, in a speech at the National Press Club.

Porter rejected widespread criticism of the proposed CIC and attacked the critics instead. He once again argued his model was superior to other proposals that imitate the powerful anti-corruption agencies in most states and territories, because it would better protect the rights of people who are accused of wrongdoing. He repeated his past attacks on other models put forward by cross-bench MPs — concerns rejected as overblown, inaccurate straw-man fallacies, and issues that could easily be addressed by public sector integrity professor AJ Brown, the architect of one of those bills.

Like the CIC’s many critics in parliament and the legal profession, O’Brien doubts it would have enough investigative oomph, on the other hand, to reassure the public that it can fully and effectively investigate serious allegations of government corruption.

The Queensland MP said he would find it hard to support the proposed agency unless it could hold MPs and senators to “the highest standard” of integrity. Draft legislation is yet to emerge but he doubts the agency described by Porter thus far will do that.

For parliamentarians, their staff, or public servants, the CIC could only quietly build a criminal case for prosecutors, so a lot of its investigations would remain secret and it would only be able to start an investigation if it had reasonable suspicion that a crime had been committed. Critics note that the case which eventually saw Eddie Obeid locked up would never have begun if the NSW ICAC worked on this basis.

The CIC would have much stronger powers to look into allegations against law enforcement officers, inherited from the Australian Commission for Law Enforcement Integrity, including the possibility of public hearings and reports. O’Brien thinks this two-speed system will not do enough to counter public cynicism.

“We need confidence in our government,” said the former police officer. “And we need the perception to be — as well as the reality — that we are doing the right thing. Now, not holding politicians to the highest possible standard, when someone else is held to a higher ethical standard, is wrong. It’s just fundamentally wrong.”

In the speech, the Attorney-General said little to justify the two-tiered model specifically but argued it was important to be very cautious with the upcoming legislation, so as not to give the agency broad new retrospective powers, or change the status quo too much.

Porter rejects the main functions of most integrity watchdogs: to investigate and expose behaviour in the public sector that clearly represents serious, systemic wrongdoing, or corruption by private interests, but which may not reach the standard of a criminal offence.

“Public servants can readily know what constitutes offences against the Commonwealth Criminal Code that relate to obtaining a financial advantage by deception, abuse of public office or money laundering,” he argued.

“The difficulty arises [when a commissioner can] investigate new offences or standards of conduct which might be called variously ‘corruption’ or ‘any other corrupt conduct’ or ‘misconduct’ or ‘serious misconduct’ or ‘improper conduct’. What behaviour falls inside or outside these terms is simply less clear.”

A key point of setting up anti-corruption commissions in other jurisdictions has been to make those definitions of bad behaviour abundantly clear, both through the wording of the legislation, which has been regularly tested at state level, and the agency’s efforts to prevent corruption. This typically involves encouraging and supporting agencies to improve their internal integrity frameworks, while providing ongoing education and training.

On top of that, most integrity commissions have what Llew O’Brien calls “real teeth” to enforce those standards of behaviour effectively, overseen by independent inspectors and parliamentary committees.

Read more: Public corruption hearings at Commonwealth level: a ‘risk worth taking’ for integrity?