Retailers and suppliers say exempting fresh fruit and vegetables from GST, as the Labour Party is proposing, would create anomalies and increase costs to companies.
Labour has announced it will make the tax change if it wins next year's election.
Leader Phil Goff says it would be a relatively straightforward policy for retailers to implement and would make a difference to families.
But the Retailers Association says the change would increase compliance costs and paperwork, especially for smaller businesses.
The Food and Grocery Council says it would discriminate against those growing vegetables to be frozen, as that produce would still incur the tax.
The Council of Trade Unions says the move would help low-income families. Its economist, Bill Rosenberg, says the loss in tax revenue could be made up by measures such as a capital gains tax that exempts the family home, or changes to the income tax structure.
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