John Howard took office in 1996 promising new heights of ministerial integrity, in the form of a tough code of conduct. His ambitions were cruelled within a year as he was forced to sack three ministers, each of whom had contributed to the "travel rorts" scandals of that era. In the same year, disgraced independent senator Mal Colston faced 28 charges of defrauding the Commonwealth by allegedly misusing his travel allowance. The prosecution was only dropped because his failing health meant he was unlikely to be fit enough for a trial.

The great surprise is that neither main party in the years since has been able to exercise enough restraint over its members to prevent similar abuses. Politicians from both sides have been tempted to use taxpayers' money to pay for personal trips or to further personal ends. Indeed, the main parties refrain from pursuing the opposing side too aggressively over such issues, knowing they are equally likely to be embarrassed by a greedy rat amid their own ranks.

Most (though likely not all) of those who cross the line are caught, because parliamentarians' spending details are published by default. It takes time, but a journalist or interested citizen committed to scrutinising politicians' trips will eventually discover those whose personal indulgences coincide with the use of public funds. The same is true for politicians' conflicts of interests, given they are required to update a public list of their financial and other interests.

Transparency alone is not enough to deter all political free-riding. Ms Ley resigned last week after widespread criticism of her use of taxpayers' funds to travel regularly to the Gold Coast, including, on one occasion, to buy a luxury apartment. Yet she insisted she had "followed the rules", and even complained that "the ongoing intense media speculation has made this an incredibly difficult week". The threat of exposure obviously failed to hone her sense of propriety.

Yet the question begs: if public registers of parliamentarians' expenses and interests allow us to scrutinise and, eventually find, the free riders among them, what of the vastly greater number of officials whose expenses face almost no scrutiny? Senior public servants control most government spending. They decide which businesses are invited to tender for government work; they sit on assessment panels for contracts and jobs. Their conflicts of interest are a far greater threat to good government. Yet the public knows almost nothing of them.Their conflicts, and expenses, are disclosed in secret to agency heads and no one else.

It's inevitable that this lack of transparency encourages indulgences, and worse. Most senior public servants are, of course, upstanding and deserving of respect; indeed, some sacrifice a more luxurious lifestyle to work in government. Others, however, will succumb to temptation. A little more public scrutiny would go a long way towards reducing the number of those who do.