They've bet billions of dollars over the years, and now the tax office is after them.

However they have an unusual ally in their fight over the $900 million tax bill.

They're the ultra-secret Aussie punters club that's plundered racetracks and gaming tables around the world.

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A group of mates, all mathematical geniuses, they bet $2 billion a year, but they're now the target of a tax probe that threatens every punter and amateur gambler in the country.

David Walsh is the unofficial leader of the gang - an art collector, philanthropist and the most visible member of this gamblers club.

Poker player George Mamacas is another member. South Australian David Steike is another poker champion, joined by Barossa wine maker and punter Ray Gatt. The biggest punter of them all is Zeljko Ranogajec - the man they call The Loch Ness Monster, who gambles a billion dollars a year on his own.

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These are just five members of the club, but there are fourteen others making tens of millions a year.

The gamblers club is Tasmania's best kept secret. It began at the University of Tasmania’s maths department when three maths whizzes got together to use their skill with numbers to make some easy money. They used the laws of probability to prove they could win.

The maths geniuses used their love of Blackjack, Keno and horses to make some serious cash.

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Their success was phenomenal - being students they played with small amounts, picking winning combinations at Keno and counting cards at Blackjack - a legal but discouraged practice.

Fellow Tasmanian punter Andrew Scott has known the members of the secret gamblers club for more than twenty years. He plays poker against them around the world and knows their tricks.

“These guys started out as Blackjack players, so they started out at Wrest Point Casino in Hobart, and they extended from Blackjack to horse racing," Scott said.

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“When the casino stopped allowing them to play Blackjack they got into horse racing, and at the end of the day they extended those principals. So they're looking for good value bets. They've decided what the probability of winning is, they've looked at what it's paying at the tote, and they bet on the bets that represent good value," he explained.

They met at a North Hobart pub and placed bets on horses. With every success, betting more and more and adding more members to their club.

“It just grew organically, and they just bet more, and more, and more, and they've won, and good luck to them."

Ranogajec won a record $7.5 million dollars in 1994 while playing Keno at a Sydney RSL, after spending six days in placing bets and working out the numbers.

Since then he's become the single biggest gambler in the world. He bets $1 billion a year, but is rarely seen. He doesn't grace the racetracks of the world he plunders, and he and his wife Shelley Wilson lead a quiet life away from the spotlight.

They now live at One Hyde Park in London - the most prestigious address in the world, where a one bedroom apartment will set you back more than $10 million and a penthouse more than $200 million.

From here the Loch Ness Monster punts mostly online, with Tote agencies offering him massive rebates for his gambling dollar.

George Mamacas and his poker mate David Steike are regulars on the world poker stage, and David Walsh has become well known for his $200 million creation - the Museum of New and Old Art or MONA.

Built just outside Hobart, at a cost of more than $70 million, and containing more than $100 million in art and books in a year it has become Tasmania's biggest tourist attraction, financed by some of the profits of the gamblers club.

Now the gamblers club has received support from a most unlikely place in its fight with the ATO. Independent MP Andrew Wilke, the man who wants to limit poker machine betting, is fighting for David Walsh and his mates.

“I'm not anti-gambling. Like just about everyone else in Australia I'm happy to place a small punt on the Melbourne cup or play two up on Anzac day. What I am also is passionately in support of a fair go for people, and if there is evidence that someone is being treated unfairly, then there is a role for me to step in and speak up," Wilke said.

"You know the ATO on four separate occasions told this particular group it was not taxable, and now they're going back and changing their mind. I mean I just think it really could cause a problem for the whole country. We could all be paying tax every time we have a punt down at the pub and win twenty bucks," Scott agreed.

And that's what's at stake. Many believe the attack on the punters club could lead to taxes on gambling winnings and even lotto wins, affecting tens of millions across the country.

“No one knows what the ATO's intentions here are. Are they just going after a very small number of very successful professional gamblers, or do they intend to extend it to all professional gamblers? Where do they intend to draw the line between professional and recreational gambling," Wilke asked.

The tax office claims the gamblers club has destroyed vital tax evidence and its gambling records. Now an audit of the successful secret club is heading to court.

Every punter in the country is anxiously waiting for the result, but few are willing to bet against the world's best gambling club.

• Gamblers Anonymous - www.gansw.org.au
• MONA - www.mona.net.au