Polson Higgs tax partner Michael Turner said the Government faced a potentially damaging negative backlash with the tax changes because despite personal tax rates dropping, GST was rising.
That meant that from October 1, petrol, groceries and the power costs for households would rise.
"Labour has consistently said that all but rich New Zealanders will be worse off by the tax changes. The Government needs to negate that."
Any tax changes would involve an education campaign because the Inland Revenue Department had to issue new tables and most people would need to choose a new tax code, he said.
Dropping the top tax rate from 38% to 33% was a major shift and opened the way for criticism that National was favouring the rich.
"Prime Minister John Key has always maintained no-one will be worse off through these changes. I am not convinced he is right. We can't all be winners. Someone has to lose," Mr Turner said.
Finance Minister Bill English said the tax changes would further boost real after-tax wages, leaving the vast majority of New Zealanders better off.
The Treasury advice showed that even when all forecast cost of living increases for the rest of the year were taken into account - including the rise in GST and a range of other factors - real after-tax wages were forecast to grow due to the October 1 changes.
The changes were the first part of the most significant tax reform package in nearly 25 years, he said.
"For ordinary New Zealanders, it will reward effort, encourage savings and help families to get ahead.
"As well as improving the incentives to work, the package tilts the economy towards savings, investments and exports and away from unsustainable borrowing, consumption and over-investment in housing of the past decade."
According to Mr English, an "average income family" would be about $25 a week better off from October 1, an average wage earner about $15 a week better off and a couple on New Zealand Superannuation about $11 a week better off.
Rises would grow as wages did.
"At all taxable income levels, the personal tax cuts will more than offset the rise in GST and low, middle and high income groups broadly receive the same proportionate increase in disposable income," Mr English said.
The second part of the charm offensive came from Social Development, Employment and Youth Affairs Minister Paula Bennett who told superannuitants they would also be better off from October 1.
A combination of tax cuts and compensation for the rise in GST meant superannuitants would have more disposable income.
A "typical" single superannuitant, living alone, would effectively receive an extra $31.26 in their fortnightly payment.
Once the increase in prices due to GST was taken into account, a single superannuitant living alone would be better off by about $8.50 a week, or about $445 a year.
Married couples would be about $560 a year better off, she said.
