First, the scandal erupted in the depths of January, whenmembers of the press gallery are desperate for copy. The daily drip-feed of revelations about Ley's various trips to the Gold Cost were journalistic manna from heaven, prompting a stream of further juicy stories about other parliamentarians. As the evidence steadily mounted and media-inspired anger intensified, Turnbull was forced to act, not only referring Ley's conduct to his department's secretary, Martin Parkinson, but also promising significant reforms to the entitlements system. At any other time of the year, the issue could have been safely parked in the non-urgent basket. But not in January.

Second, though the general issue was parliamentarians' expenses, Ley was a minister as well as a member of Parliament. As such, she was subject to the ministerial code of conduct that is overseen by the Prime Minister. This allowed Turnbull to step in and directly involve his departmental secretary. Parkinson's report on Ley's conduct has not been released. But we may presume he took a careful, objective view of the issues and the code. He will then have played an active role in shaping the eventual recommendation for a new independent authority to oversee MPs' expenses. If Ley had been an ordinary backbencher, however, such a strong executive response wouldn't have been so easy. Members of Parliament don't have their own code of conduct and are accountable only to Parliament. Issues of MPs' behaviour are caught up in either the partisan conflict of adversarial politics or the cosy consensus of self-serving privilege, neither of which are conducive to the sensible assessment of ethical procedures. The focus on a minister fortuitously helped Turnbull impose a new level of external scrutiny on all MPs.

Third, the Prime Minister and his advisers had plenty of relevant material to draw on. The government was already developing a response to an independent review of parliamentary entitlements co-chaired by John Conde and David Tune. This review was established by then prime minister Tony Abbott in August 2015 as a response to the "choppergate" scandal involving then speaker Bronwyn Bishop. The review team produced its final report a year ago. Its main recommendations were to change the terminology from "entitlements" to "work expenses'", to introduce a simplified principles-based system for assessing expenses, and to require more public transparency.

In addition, the government could look to the experience of Britain, which had its own parliamentary expenses scandal in 2009 over blatant misuse of housing allowances and established an independent parliamentary standards authority. In his press conference, Turnbull referred explicitly to Britain as a source of useful precedents. Britain also figured prominently as a model in a 2011 discussion paper by the House of Representatives' privileges and members' interests committee on a draft code of conduct for members of Parliament. That paper canvassed the possibility of an independent parliamentary integrity commissioner overseeing a members' code of conduct on British lines, issues that were championed by independent MPs Rob Oakeshott and Tony Windsor as part of their compact with Labor prime minister Julia Gillard. Like so many promising initiatives on parliamentary ethics, it eventually went nowhere, withering under the weight of major-party indifference. But it laid some useful groundwork and helped keep the issue alive.

Turnbull has already announced that the authority will have a governing board including an auditing expert, someone with experience in remuneration (for the time being, the Remuneration Tribunal president), a former judicial officer and a former MP. It is to be a "compliance, reporting and transparency body" that will "monitor and adjudicate all claims by MPs, senators and ministers". The Prime Minister also indicated reforms to the administration of the system to allow monthly disclosure of parliamentarians' expenses. (He also strongly endorsed the recommended change in terminology from "entitlements" to "expenses".)

The new expenses regime, we can presume, will follow the simplified principles-based approach recommended by Conde and Tune. Contrasting with a reliance on specific and detailed rules, the principles-based approach is now standard practice in most codes of professional ethics, not just for politicians. The situations in which particular ethical problems arise are so complex and varied that they can't be covered by detailed rules and prescriptions. Ethical professional judgment involves a few broad principles (e.g. equity, accountability, honesty), which are then used to guide a careful assessment of a particular issue and to justify an eventual decision. Responsibility for the decision lies with the person making it and can't be shifted on to someone else.

Individual ethical judgment of a particular situation is crucial in the case of parliamentary entitlements, where the relevant factors are so complex. The principles may be clear (e.g. claim expenses only for work performed predominantly in the public interest) but their application is far from straightforward. The key point is to make individual politicians responsible and accountable for their own decisions and to prevent them from hiding behind a set of bureaucratic rules and precedents that may or may not be appropriate to their case. For too long, politicians have been able to escape responsibility by claiming that a given expense is "within the rules" regardless of its ethical propriety.

In this respect, the most important element of the new structure is probably that it removes oversight of expenses from the Finance Department. So long as public servants are asked to authorise politicians' expenditure, they will inevitably fall back on bureaucratic rules as a means of self-protection. Public servants can't be expected to evaluate individual ethical judgments made by politicians, especially ministers. They are naturally more comfortable applying supposedly cut-and-dried rules and precedents. The present system therefore represents a collusive compact between public servants who are unwilling to ask hard questions and politicians who are unwilling to answer them. A change in institutional structure requiring politicians to justify their spending to an independent authority will help force a change of attitudes and practice.

Equally important will be the improved transparency requirements, giving the public, including the media, full and prompt access to politicians' individual expenses. The reforms arise out of public disgust at the extent to which politicians are benefiting themselves at the public expense. The clinching argument is always that a particular level of expenditure doesn't meet the so-called "pub test", i.e. the ethical judgment of the average citizen. (Average citizens, of course, are not to be found only in pubs!) In justifying their expenses, politicians will need to have in mind the need to satisfy a sceptical and cynical public.

On its own, the expenses authority is unlikely to enforce these standards. Drawn from the great and the good, who do their own share of wallowing in expenses, it will inevitably have a more complaisant attitude than the average citizen ("the chairman's club test" is much softer than "the pub test"). The authority's decisions will therefore need to be regularly exposed to the blowtorch of tabloid outrage.

The main weakness in the authority as currently proposed is its limited scope. Focusing solely on parliamentarians' work expenses, it overlooks a number of other issues usually considered relevant to promoting ethical standards among parliamentarians. In Britain, for example, the office of the Parliamentary Commissioner for Standards is responsible for overseeing the register of members' financial interests. It also monitors the operation of the House of Commons code of conduct. Australian parliamentarians still have no code of conduct, an omission that is becoming steadily more anomalous. Codes of conduct can be oversold as mechanisms for improving ethical standards. But as the Ley case reminds us, they provide useful standards for assessing the behaviour of those suspected of crossing the line.

Extending the new authority's purview and introducing a code of conduct can wait for another day and another set of scandals. At least, with the authority in place, future prime ministers, when cornered over the issue of politicians' impropriety, will have an obvious solution at hand: strengthen the authority.

Advocates of a federal anti-corruption agency have been quick to link the crisis over parliamentarian expenses to the wider anti-corruption cause. Greens leader Richard Di Natale, for example, welcomed the new authority as "a positive step" while signalling his party's intention to legislate for a national anti-corruption watchdog, within which the authority would sit. The move for a "federal ICAC" is certainly gaining ground, with Labor softening its previous indifference. The sight of politicians flagrantly abusing their travel allowances will have strengthened public support for stronger measures against corruption.

But a government-wide anti-corruption agency would need much bigger fish to fry than politicians engaging in minor rorting within the law. Only when ministers or senior bureaucrats are caught taking bribes or engaging in other forms of serious illegality will the case for such an agency become compelling. Supporters of a federal ICAC should be hoping that some accountability agency uncovers major wrongdoing in a Commonwealth agency. If the scandal were to break in January, so much the better.

Richard Mulgan is an emeritus professor at the Australian National University's Crawford School of Public Policy. richard.mulgan@anu.edu.au