The government's previously strong position on its bank tax started to wobble yesterday, with plenty of help from the banks and Labor. It produced two separate but closely related issues: how much money the levy would raise, and the tax deductibility of the government's proposed levy.

The levy is tax deductible -- that much was confirmed by the government, after the banks began releasing their estimates of their likely liabilities. But had the government taken that into account when it calculated it would reap $6 billion over four years from it? Yes it had, the government eventually, after some confusion yesterday afternoon, insisted. The impact of the levy in the first year (that's 2017-18, starting in just over a month) would be felt in company tax returns for 2018-19, which is why the levy revenue estimates varied across the next four years in the budget.

But when you toted up the banks' self-admitted liabilities yesterday, you were left with a total take well short of the $1.6 billion expected next year -- not even a billion dollars from the big four banks, with Macquarie, which has kept a low profile on the whole issue, yet to 'fess up on its impact.