Some of Australia's best and brightest have joined a global campaign for a so-called Robin Hood tax.

The tax, technically known as a Tobin tax, would take a tiny levy from each of the trillions of financial transactions that happen every millisecond on global markets.

In Australia, Reverend Tim Costello and comedian Julian Morrow are among those urging action.

Advocates say the tax would reduce damaging financial speculation and create a giant pool of money that could be used to feed the hungry and combat global warming.

In a YouTube campaign video, actor Bill Nighy plays a reluctant banker who is gradually sold on the merits of what has been dubbed the Robin Hood tax.

It is a modern version on the old theme of taking from the rich to give to the poor, a minute tax on all financial transactions other than routine banking by the public.

Advocates say the tiny tax would raise hundreds of billions of dollars to combat global warming, reduce poverty and repair the battered finances of governments after the global financial crisis.

Ethicist Peter Singer is among other prominent Australians who have joined the global campaign.

"We're talking about a really tiny tax. We're talking about, not 1 per cent, not a tenth of 1 per cent, but a 20th of 1 per cent," he said.

"Obviously I'm most interested in the fact that it can raise funds from all the world to help the world's poorest people and to help pay for the damage that we're doing to them through climate change.

"But there's also an economic argument that it will reduce the volatility of international markets.

"That is, instead of huge billions of dollars being thrown around the world for a few moments here and there to earn a tiny profit, a very, very small tax would have an inhibiting effect on that."

'Naive' plan

A tiny tax that curbs the speculation that makes mega-profits for banks like Goldman Sachs and helps make poverty history: it sounds too good to be true, and critics say that is because it is.

Dr Sam Wylie, a senior fellow at the Melbourne Business School, says the idea is being promoted by well-meaning but naive people.

"It's not a very good idea. It has the potential to do quite a lot of damage and very little good," he said.

"There's certainly some great people involved. I know that Tim Costello and Peter Singer, for instance, are tremendous Australians and powerful intellects promoting this idea.

"But they're not finance people. They're great in their own fields but I really feel that they don't understand the complexity of the modern financial system."

Dr Wylie says the tax would undermine legitimate attempts to hedge risk.

"Most of what's going on in transactions in financial markets is hedging and not speculation," he said.

"If you're a company like Qantas, [that] faces a lot of risk associated with the oil price and so they put in place a set of hedges to control the oil price risk that they face.

"A Tobin tax says if you want to adjust your hedging position, you have to pay for that.

"A Tobin tax says to National Australia Bank, who borrow money overseas in US dollars, bring it to Australia and lend it to Australian households to build and buy houses.

"If you want to hedge your foreign exchange risk, which they simply have to do, if you want to change your hedging position, you have to pay for that.

"So it actually has the potential to do a lot of damage. People think that most of what's going on in these transactions is really to do with wild speculation. But that's just not right."

Hedging a gamble

But University of NSW professor and expert on banking and finance law and policy, Ross Buckley, disagrees.

He says what some call "hedging" is really a financial punt that lasts only a few seconds.

"That's simply not true. I mean, nobody hedges for two seconds, right? Nobody hedges for nanoseconds," he said.

"A substantial proportion of transactions in modern financial markets are for less than a second. The asset is bought, held and sold - the total time period is less than a second. That's not a hedge, that's a gamble, like at the race track.

"In Australia [a Tobin tax] would probably create something upwards of $1 billion a year, maybe $1.3 billion or $1.4 billion. Globally, maybe $350 billion to $400 billion.

"What you do with it is a political decision, but the way the lay of the land is at the moment is probably about half of it goes to the governments that are collecting it, in an ideal world, not more than half, and the other half gets used to global public goods.

"Even if a quarter of this tax went to global poverty, that would be as much as the total aid budget of all rich countries today.

"If another quarter went to climate change adaptation, that would be $100 billion a year."