Imagine if you decide one day to use a new accountant. He promises that everything will be done on time, and you'll get a hefty tax refund.

You hand him the group certificate from your employer. He says the figures on it are not worth the paper they're written on. You point out that he'll have to use some estimate of your income. He responds, ''Don't worry, we won't be adding up your tax return this year.''

You ask about that promised refund. He shows you a draft of the return. It shows the same deduction claimed in two places. As tax day approaches, the accountant keeps promising to do your return in ''plenty of time''. But with three weeks to go, you're starting to fret.

This is what it would look like if Tony Abbott and Joe Hockey ran an accounting firm.

The story to date: In 1998, John Howard and Peter Costello came up with the notion of a Charter of Budget Honesty. Before the election, Treasury would release an independent budget statement (the Pre-Election Fiscal and Economic Outlook, or PEFO) and the resources of Treasury and the Finance Department (and now the Parliamentary Budget Office) would be available to all parties to cost their policies. Based on this, both government and opposition can produce costed policies.

While the Charter of Budget Honesty was a Liberal invention, it failed to comply with it at the last election. Post-election analysis by Treasury uncovered an $11 billion gap in the Liberals' costings.

The Liberals had claimed in that campaign that their costings had been audited, but the firm who did the job was fined for professional misconduct. This time, the Liberals have a costings gap that is, on their own admission, $70 billion.

They have pledged to give fringe benefits tax breaks back to people who do not use the car for business purposes, and to restore the private health insurance rebate for the richest Australians.

Their proposed paid parental leave scheme offers nothing more to a worker on the minimum wage than the current scheme, but provides more than $70,000 per baby to those who earn the most. Each of these policies are skewed towards the most affluent. Together, they constitute a massive cost to taxpayers.

The problem for the Liberals is that they have spent the past three years saying ''no'' to Labor's sensible savings measures. As a result, they are now in the position of promising to spend more, tax less, and pay off debt faster; oblivious to the fact that it isn't mathematically possible to achieve all three.

In the first week of the campaign, Abbott's sole policy announcement of substance was a cut in the company tax rate. Spruiking it, he claimed that it would be paid by savings he'd already announced. But those savings have already been taken by the commitment to give a tax cut to big miners and big polluters. Once that's paid for, Abbott has nothing in the kitty.

For Australian families, Abbott's costings gap means tax increases or service cuts amounting to thousands of dollars for the typical household. Most likely, that means a reduction in the quality of schools, hospitals, and family payments. Over recent budgets, Labor has made tough decisions affecting the public service. Anyone who knows our federal public service is aware that any ''fat'' is long-gone.

If elected, the Liberals are likely to maintain the efficiency dividend, and to add on top of it their policy of getting rid of 12,000 to 20,000 Canberra public service jobs. This would be devastating for our city, and yet it would be only a small fraction of the costings gap that Abbott needs to make up. Even after slashing the Canberra public service, abolishing the schoolkids bonus and scrapping trade training centres, Abbott is still left without enough money to pay the bills. So will the Liberals provide some honest answers?

In July, Hockey said that if the PEFO figures were the same as those in previous budget updates, ''PEFO won't be worth the paper it's written on''. Last week, he went further, saying of Treasury costings, ''These numbers just look stupid so we won't be adding up our policies.'' These were such extraordinary statements from a would-be treasurer that even Abbott contradicted them, saying, ''The budget bottom line will be there for everyone to see.'' Yet while Abbott is promising that his full suite of policies will be out in ''plenty of time'' before the deadline, he is increasingly looking like a teenager preparing to pull an all-nighter to get the assignment done.

And now, the jig is up. In PEFO, Treasury has confirmed what the government has been saying for months - that the forecasts in the budget and its updates reflect the best estimates of the experts.

The Liberals no longer have the excuse that they're waiting for PEFO. Now, let's hope they're finally willing to level with the Australian people about their tax rises and service cuts.

Because as much as Abbott likes to talk about the laws he wants to repeal, he can't repeal the laws of mathematics.

Andrew Leigh is the federal member for Fraser. andrewleigh.com