In budgeting it's known as the magic asterisk, the get-out-of-jail-free card. If it looks as though you'll be in the red, you simply add in a figure for general unspecified savings and, magically, your problem's solved.

Traditionally, it's extra, unspecified savings from cracking down on tax avoidance. This time it's welfare overpayments, many of them unintentional. Newstart recipients and families are asked to estimate their incomes in advance. Often they do better than expected and find themselves owing money back. With a bit of effort it ought to be possible to collect a bit more.

But the Coalition's difficulty in arguing that it can collect an extra $2 billion more is that it has already announced plans to collect $5.7 billion more. They named their 2015 packages: "strengthening the integrity of welfare payments" and "enhanced welfare payment integrity".