These days Opposition Treasury spokesman Joe Hockey steps 25kg lighter and appears to be losing more weight by the week. This is a startling transformation for the person John Howard once described as a “big bear of a man” and whom Labor regularly taunted as being “Sloppy Joe”.

Hockey is transforming in more ways than one, with his focus firmly fixed on the job of treasurer should the Coalition win the September 14 election. Few of his predecessors would be better prepared for the role than the member for North Sydney.

After nine years as a minister, including for financial services, a place on the Howard government’s expenditure review committee and a previous career as a banking and finance lawyer, Hockey has defied his detractors to emerge as a politician with prescience.

Three years ago Hockey warned that federal Labor’s claims of a budget surplus by 2012-13 would never be met. Commentators howled him down for questioning Treasury forecasts but Hockey remained persistent. History proved him right when, just days before Christmas last year, Labor’s “rolled gold”, “come hell or high water”, “guaranteed” surplus was no more.

It was a devastating blow for Julia Gillard’s government, which had staked much of its political reputation on reaching a surplus after four of the biggest deficits in Australian budget history.

But any I-told-you-so vindication for Hockey was hard to celebrate, coming as it did just hours after he had undergone bariatric surgery.

For a person who lives so much in the public eye, having trekked the Kokoda Track and climbed the highest mountain in Africa, this dramatic solution to weight loss was a decision Hockey took in secrecy with his family. Few staff were told. Close friends were kept in the dark. Even Tony Abbott didn’t know.

A politician’s constant dilemma is a poor diet. Irregular hours in Canberra combined with meals on the run, relentless airline travel, political forums and speech-making brings with it inevitable access to an abundance of food and lack of exercise. Much like an alcoholic left alone in a pub, it is a job where temptation can often be too great.

Fat boy taunts have long been stock in trade for the anti-Hockey forces in federal parliament. And while Hockey usually gives as good he gets, what finally prompted him into the drastic decision to undergo surgery was an innocent question from his daughter. “Will you be alive when I get married?” she asked as she watched a replay of the royal wedding. It startled Hockey into initiating fundamental change.

Hockey had regarded himself as relatively fit. From a young age he was forced into exercise, having to beat chronic asthma. He was a morning and night trainer under swimming legend Forbes Carlile. But in later years, competition rugby, long treks and daily exercise were not enough to beat the weight challenge.

Hockey confides that the examples of billionaire James Packer and advertising guru Harold Mitchell, who had both undergone similar irreversible surgery, encouraged him to proceed with a radical procedure that removed almost 80 per cent of his stomach.

Although the operation is not a panacea, there is no turning back; it is surgery for life. It requires serious lifestyle changes in eating and exercise.

So for Hockey it has meant dramatically reduced portions of food, plus a daily program of 1 1/2 hours of training.

Often regarded as an “effortless politician”, Hockey has charm and enjoys good company but he can also be ruthless. When in 2004 as Minister for Human Services Hockey took over Medicare, it was losing $20 million a year. Hockey ordered the chief executive to cut 400 staff from head office. Medicare then turned the corner with much reduced costs.

Hockey was also one of only two ministers to have the courage to front John Howard before the 2007 election, advising him to retire for the sake of the party before an inevitable election defeat.

As Hockey begins an arduous year of election campaigning, it is becoming evident that his energy levels are high, and increasing. Ahead is the mother of unlosable elections. But he isn’t getting ahead of himself by suggesting that the Coalition is over the line.

In a recent visit to the knife-edge seat of Lindsay in western Sydney, cameras recorded him being mobbed by voters, many of whom claimed to be Labor supporters saying it was time for change. Such was the reception that, in an hour, he barely covered 50m of a vast shopping centre.

Some may reduce this popularity to the “Sunrise factor” but that is unfair. An appearance once a week on a morning TV show doesn’t make a prominent national profile. Nor does it make a treasurer.

The new slim Hockey still cuts a formidable figure in the Coalition ranks. His economic updates to the shadow cabinet and party room are carefully absorbed as he warns of the mountain ahead to restore business and consumer confidence. Labor’s carbon tax and mining tax revenue disasters leave a massive budget shortfall and he regularly tells colleagues that it cannot be fixed overnight.

Hockey is seldom afraid to use self-deprecating humour to get a message across. One colleague recalls Hockey recently telling Liberal Party MPs: “If I am prepared to cut my own waist like this, imagine what I am prepared to do to Labor’s waste!”

While the Coalition claims it has as many as 80 costed policies ready to roll, it is waiting for the May budget to get a better handle on the nation’s finances. The government will reveal how it intends funding the National Disability Insurance Scheme and the Gonski education package – between them $14 billion a year commitments.

The Coalition has little choice but to back both measures, but the Hockey mantra remains one of caution: “We will not make promises we cannot keep”. Five years of opposition have given Hockey a different perspective on politics. He admits telling friends that he reached a nadir in the dismal days after the 2007 election loss but fought back to remain committed to public life.

Hockey’s personal transformation may inspire others in both politics and the wider community. But what he does know is that there is no turning back – personally and politically. In seven months he will be either responsible for a $360bn budget and a $1.4 trillion economy or yet again reassessing the dark days of opposition.

Emeritus professor of history and politics Ross Fitzgerald is the author of 35 books, including ‘My Name is Ross: An Alcoholic’s Journey’.

The Weekend Australian, March 2-3, 2013, Inquirer p 17.