Rich individuals and their families have at least $US21 trillion of hidden financial assets in secret tax havens globally, latest research suggests.
A study has estimated the extent of global private financial wealth held in offshore accounts at the end of 2010 - excluding non-financial assets such as real estate, gold, yachts and racehorses.
The research was carried out by James Henry, former chief economist at consultancy McKinsey. It was commissioned by lobby group Tax Justice Network, which campaigns against tax havens.
Mr Henry used data from the World Bank, International Monetary Fund, United Nations and central banks, the BBC reports.
He says the figure his study came up with is actually conservative, and the true scale of money in offshore tax havens worldwide could be $US32 trillion.
Mr Henry says the super-rich move money around the globe through a "bevy of professional enablers" in private banking, legal, accounting and investment industries.
The report highlights the impact on the balance sheets of 139 developing countries of money held in tax havens that is put beyond the reach of local tax authorities.
Mr Henry estimates that since the 1970s, the richest citizens of these 139 countries had amassed $7.3tn to $9.3tn of "unrecorded offshore wealth" by 2010.
He says there are now nearly 80 financial havens in the world, including in Samoa and Vanuatu.
A partner at PricewaterhouseCoopers in New Zealand, Geof Nightingale, says all countries are making great efforts to ensure everyone is paying their fair share of tax.
Mr Nightingale says says cash-strapped governments, including New Zealand, are looking for every last dollar they can find.
The IRD has its high net worth individual programme to make sure the assets of those with $50 million or more are being properly managed.
"People with those high levels of wealth tend to have quite complex affairs."
Progress here but 'more needed'
The Institute of Chartered Accountants says New Zealand is making progress fighting tax havens potentially siphoning off tax money, but more work is needed.
Tax director Craig Macallister says Information Exchange Agreements have been signed by many countries, including New Zealand, the Cook Islands and Vanuatu.
He says the agreements don't stop people investing in potential tax havens but do let developed countries know where the money is going.