If you're a New York multimillionaire, you now have another incentive to stay alive. A change this month in New York's estate tax, which was billed as tax relief for the wealthy, contains a hidden wrinkle that could leave some multimillionaires with a much bigger surprise tax upon their death. Certain estates could even wind up with a tax rate of 164 percent on portions of their estates, according to one tax expert. The changes were intended to ease the tax bill for wealthy New Yorkers and prevent them from fleeing to lower-tax states. A report from the Tax Foundation found that New York had the highest tax burden in the country as a percentage of state income. It found that New Yorkers spent 12.6 percent of their per capita income in 2011 on state and local taxes. Read MorePitfalls of reverse mortgages may pass to heirs



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Instead of lowering the tax bill, the tax changes could hit some estates with much higher-than-expected rates.

"It's nonsensical," said Kevin Matz, an accountant and attorney in White Plains, N.Y. "The governor said this is about making New York a better climate for the wealthy. It's had the opposite effect." Read MoreGates to China's rich: Give more

On its face, the new law seems like tax relief. Under the previous law, New Yorkers paid estate taxes of 3.06 percent to 16 percent on the value of estates over $1 million. The new law raises that exclusion to $2.062 million this year and gradually increases it to more than $5 million by 2017. But because the law also phases out certain credits related to federal taxes, people who have estates valued just above the $2 million threshold could get massive estate tax bills. An analysis by U.S. Trust found that a New York resident who dies today with a taxable estate of $2,165,625 could have to pay an estate tax of over $112,050. That represents a tax of over 100 percent on the value of the estate over $2,062,000. Read MoreWhen your kids have private-jet envy

It gets worse in a few years. Matz said that assuming that the exclusion rises to $5,250,000, a New Yorker with a taxable estate of $5,512,500 would have to pay an estate tax of $430,050. That's a marginal tax rate of 164 percent on the value of the estate above the exclusion.