One tax expert says a wealth tax is unworkable and unnecessary.

The Green Party wants to introduce a wealth tax of 2.5 percent on assets worth more than 2 million.

This would extend to assets worth more than $4 million owned by a couple.

OliverShaw tax advisor Robin Oliver says it's difficult to calculate one's wealth.

"Some things you can value- listed company shares. But outside of that with small businesses, everything else, artwork, it's a matter of judgement and you don't know until someone fronts up with the cash."

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