|Other names||John Wilson|
Ranogajec was born in Hobart, Australia, in a family of Croatian immigrants. He was a Commerce and Law student of the University of Tasmania, studying tax, finance and banking, when he began card counting in Blackjack at the Wrest Point Casino. While there, Ranogajec met his future wife and business partner, Shelley Wilson, who was a full-time employee. He subsequently relocated to the mainland in New South Wales where he transferred his studies to the University of NSW in Sydney from which he eventually dropped out of to concentrate fully on advantage gambling.
According to professional blackjack players, Ranogajec was "one of the most prolific and innovative advantage players of all time". His starting bankroll was allegedly "a few hundred dollars", through which he won "millions". Ranogajec worked with Alan Woods in the 1980s.
Eventually, Ranogajec was banned from most casinos in Australia, starting with Wrest Point, the Jupiters Casino on the Gold Coast, and then, after he moved overseas, in the United States as well. Ranogajec, by that time, was already moving to other areas of advantage play, in any area of gambling where an advantage could be identified.
In 1994, Ranogajec reportedly won a $7.5 million Keno jackpot at the leisure and entertainment complex North Ryde RSL Club, of New South Wales, after reportedly betting "significantly more than $7.5 million" to win it but coming out ahead on account of the additional, smaller prizes awarded along the way to the jackpot.
Ranogajec, according to insider accounts, deployed, for his betting activities on horse races, a strategy combined of specific factors: identifying betting opportunities with as high liquidity as possible, meaning betting pools with significant money being bet by the regular gambling public; deploying a "highly sophisticated betting system"; identifying small margins, on which he bets significant amounts of money; and, importantly, closing deals with bookmakers, including industry leader TabCorp, for significant rebates on his bets. The latter has created controversy in the horse betting world, with many punters complaining about the alleged deal. A TabCorp spokesman stated, in response to the media reports, that the corporation "investigated the incidents" and found "no evidence of illegal activity", but reminded the betting public that "the offering of tote-odds betting products by corporate bookmakers has inherent risks of pool manipulation [by large bettors]."
Ranogajec reportedly accounts for 6–8% of Australian bookmaker TabCorp's $10 billion annual revenue. His betting on Betfair is believed to account for one third of the company's Australian operations.
Ranogajec has gradually organised an extensive network of spotters, analysts, bettors and administrators, directing activities from office space he is allegedly occupying in the Fox Sports building at 235 Pyrmont Street, in Sydney's inner suburb of Pyrmont. It is known that he contracts many companies to provide information and does not employ anyone directly. However indirectly due to his information requirements, to gain the edge, he provides employment to over 300 people in Australia.
Ranogajec's success has been revealed to be based upon favorable discounts and rebates that he has negotiated with betting pool operators. In 2011 it was reported that the commercial failure of Tote Tasmania was partly due to the large rebates that Ranogajec had received on his betting turnover.  Subsequently, the business was purchased by Tatts Group. According to Ranogajec's own testimony in a court case involving a former bookmaking associate, the plan was: “You bet to lose, so that you actually turn over more money and the win comes from the rebates … If you bet $100 and lost $5, but you get a 10 per cent rebate, you still make 5 per cent.” Ranogajec estimated he'd earned about $52m over a three-and-a-half-year period betting on US races, but just 15% of that sum came from picking winning horses. 
The "Punters Club" Syndicate
Ranogajec headed a group known as the Punters Club, consisting largely of math-inclined gambling geeks, reportedly including high-stakes poker player David Steicke. The Club is believed to have contributed over $1b annually to Aussie TAB outfits’ turnover but generated significant controversy in 2011 when it was reported that Tote Tasmania was offering lucrative rebates to such whales while slashing odds on rank-and-file punters to balance the books. 
By most accounts, Ranogajec is secretive and does not give interviews. According to some reports, there could be some intentional confusion as to his real name, whereby he'd be using his wife's surname, registering as John Wilson for investment or gambling endeavours.
The secrecy extends to his exact wealth, on which only speculations are being made, as he has never been mentioned in a "Richest Australians" list. A reporter claims that he was able to contact a relative of Ranogajec from his father's side, who asked not to be named, and who told him that Ranogajec is a "multi-billionaire". The reporter ostensibly met Ranogajec in person, while the latter was in Mosman, and asked him if he was indeed the world's biggest punter. Ranogajec supposedly replied "I believe that's absolutely untrue" and assured the reporter that all the talk about the magnitude of his betting and wealth is "all just a big exaggeration".
Tax audit controversy
The ATO has looked at his activities on numerous occasions over the last 25 years and on each occasion has concluded his activities did not constitute a business. Due to the level of his success, the ATO had decided to probe again in 2008. The ATO's current position is that betting and gambling wins are not assessable unless you are carrying on a business of betting or gambling and is contained in published rulings for gambling syndicates in Australia. His business partner David Walsh believes that he does not owe them money as gambling on horse racing has never been a taxable source of income in Australia. Tax experts following this case have said if the ATO has now changed its position on horse racing gambling, taxing retrospective wins is unjust and imminent for a high court challenge and could potentially open up a can of worms for every Australian who has won on horse racing in the past.
After months of debt claims by the ATO, a settlement has been agreed upon with Zeljko and other members of the punter's club, late October 2012. After a court-ordered mediation session between parties. The Tax office deal remains confidential but it understood to relate to the years 2004–2011 only.
Since leaving Australia, Ranogajec is now based in Europe, mostly UK and the Isle of Man. He is a consultant to Newfield Limited,  which describes itself as an international racing and sports event company. 
The "Punters Club" Syndicate
Ranogajec headed a group known as the Punters Club, consisting largely of math-inclined gambling geeks, reportedly including high-stakes poker player David Steicke. The Club is believed to have contributed over $1b annually to Aussie TAB outfits’ turnover but generated significant controversy when it was reported that Tote Tasmania had offered lucrative rebates to such whales while slashing odds on rank-and-file punters to balance the books. 
Downfall of Tote Tasmania
In 2012 Ranogajec was named as being the leader of a gambling syndicate which had profited from a rebate arrangement with Tote Tasmania Pty Ltd which was a Tasmanian state-owned company. The profits paid to Ranogajec and his partners virtually wiped out the profits of the Tote Tasmania Pty Ltd, to the extent that Treasurer of Tasmania was reduced to selling off the entire business to Tabcorp Holdings 
In the United Kingdom, Ranogajec is a co-founder of Colossusbets Limited  along with Bernard Marantelli.  Colossus Bets had announced a new venture partnership with the consortium Britbet comprising 55 British racecourses to operate horse betting racing pools and become a "major competitor in Tote service". On 12 June 2018 it was announced that the Britbet project with Colossusbets as a partner, had been terminated.
- McClymont, Kate (17 May 2018). "Meet the Joker: the Australian who is the biggest gambler in the world". The Sydney Morning Herald. Retrieved 21 May 2018.
- "Australian Croat Betting Billions Annually", Slobodna Dalmacija, 3 November 2004 (in Croatian)
- "You can bet on The Joker", Herald Sun, Australia, 13 February 2010
- "2011 Blackjack Ball" Blackjack Insider
- Arrold, Tony (1 February 2008). "Gambler more than broke even". theaustralian.com.au. Retrieved 7 March 2018.
- "The world's biggest punter is Zeljko Ranogajec, and he's an Australian", The Daily Telegraph, Australia, 13 February 2010
- "High-flying punter on kickback", Herald Sun, 20 May 2011
- “Zeljko Ranogajec is named as the World’s Biggest Gambler” Casino Promo, 2 December 2010
- "Taxman targets the king of punters"/ The Australian, Australia, 24 December 2011
- Contact information, Fox Sports Australia
- "Zeljko Ranogajec believed to be Australia's biggest punter", Courier Mail, 13 February 2010
- Delvecchio, Jerry. "Zeljko Ranogajec, The worlds (sic) biggest sports bettor! – The Worlds (sic) Greatest Gamblers". worlds-greatest-gamblers.com. Retrieved 14 September 2015.
- Private Rulings ATO Private Rulings example
- "Punters club cuts a deal with ATO"/ The Financial Review, Australia, 20 October 2012
- "On Punters club, Tax Office backs $1.8bn loser"/ The Financial Review, Australia, 27 October 2012
- "Aussie is world's biggest gambler". NewsComAu. 17 May 2018. Retrieved 1 June 2018.
- Barber (12 June 2018). "New pool-betting project Britbet on hold as talks continue with the Tote". Racing Post. Retrieved 20 June 2018.