Joe Hockey's claim was found to be mostly false.

Doing it two years earlier "could have a $15 billion impact on the government's revenue over the forward estimates", according to the Coalition's climate change spokesman Greg Hunt, who provided the figures used by Mr Hockey.

"Forward estimates" is budget terminology for the current financial year plus the next three. But this impact would be over two years because the fixed carbon price was set to end in July 2015 in any event.

Mr Hunt's office said it arrived at the $15 billion figure using Table 3.2.7 of the Clean Energy Regulator's budget statement, which showed the government expected to receive $8.34 billion from the carbon tax this financial year and $9.27 billion the next. The cost to the budget from a much lower carbon price of $6.50 would be $6.1 billion this financial year and $6.9 billion the next, by that reasoning. It was not quite the $15 billion claimed by Mr Hockey, but it was close.

But the figures Mr Hunt relied on applied to gross income from the sale of carbon permits. The net income, listed in the same table, was much lower because more than 40 per cent of the permits were given away. The gross income was a notional rather than actual figure, since a huge chunk of it would never be received. And what will not be received cannot be lost.