The Council of Trade Unions says hundreds of millions of dollars in taxes are not being paid by the country's wealthiest individuals and changes to the system are needed.
Inland Revenue Department figures show only half of all wealthy individuals worth more than $50 million are paying the top personal tax rate.
The rest said they were earning less than $70,000 per year and so were not paying tax at the highest rate.
Personal income of more than $70,000 per year is taxed at 33 cents in the dollar.
CTU economist Bill Rosenberg said that rate is low by international standards and the richest people in the country should be able to afford it.
He said a capital gains tax, a financial transactions tax and stopping trusts from being used for tax purposes could recover the lost money.
About 250 people are worth in excess of $50 million and the IRD surveyed 184 of them.