Whether he realised it or not, Joe Hockey yesterday committed a Coalition Government to slashing spending by more than $40 billion.

Hockey told The Australian there was “a strong argument that government spending as a proportion of GDP should be no more than 24 per cent”.

This year it will be 28.7% of GDP.

The likely first Hockey Budget in the event of a Coalition victory would be 2011-12, when on current Budget figures, spending will 27.4% of GDP. The Australian suggests this would require cuts of $14 billion. But lopping nearly 3.4% of GDP off revenue will require a reduction from the forecast $356.4 billion to about $314 billion — or around $40 billion.

Factor in a better-than-expected recovery between now and then and maybe it could be down to a mere $30b.

Talk about your horror budget.

Hockey, displaying commendable realism, says it will take a number of years, which might mean several austere budgets in a row.

The last Government to expenditure achieve 24% expenditure: GDP was the Hawke Government at the end of the 1980s when Paul Keating was in his pomp and Peter Walsh was Finance Minister.

With all respect to Hockey, he’s no Keating, and I suspect even Helen Coonan would admit she’s no Peter Walsh, the ultimate Dr No who made fiscal pain briefly fashionionable in Australian history’s most reformist Government.

Peter Costello managed the figure on one occasion, but it was because of the switchover to accrual accounting in 2000. Costello had managed the impressive task of cutting the figure from 26.2% in 1995 to first 25.6% and then 24.3%. No Government, one suspects, could ever manage more than what Costello and Fahey managed in those first two budgets, meaning Hockey will need a long series of slashing cuts to ever get within cooee of his self-selected goal.

Yesterday wasn’t a good day for Hockey. Lindsay Tanner had a go at him for tweeting during Question Time (good for you, Joe – it helped liven up one of the most deathly boring Question Times in this Government, which is saying something, so please keep it up) and Hockey then went on Sky to bag Gordon Brown and Barack Obama, declaring “many people were starting to question” Obama’s handling of the US economy.

That should make life interesting in the event Hockey become Treasurer and has to deal with Gordon Brown and his chancellor, or develop a close relationship with Tim Geithner. Especially given Obama probably hasn’t forgotten which side of Australian politics labelled him the terrorists’ choice during his campaign.

Hockey must have been in an expansive mood — on Twitter, he had suggested everyone at the G20 Finance Ministers’ meeting from which Wayne Swan had just returned was left-wing. Angela Merkel and Nicolas Sarkozy might have been alarmed to discover the true leanings of their finance ministers.

Hockey occasionally shows some policy grunt, but why is it difficult to avoid the impression that the many people criticising Hockey as a lightweight might be right?

Expect Hockey to cop plenty in Question Time today. That 24% figure is about to burnt into his admittedly thick hide by the Government.