A member of the Tax Working Group says New Zealand should not bank on a capital gains tax to boost revenues, as it's too volatile.
There is speculation that the Labour Party plans to make a capital gains tax the centrepiece of its economic policy at a rate of about 15%, with the family home exempted.
Labour will officially launch the policy next Thursday.
PricewaterhouseCoopers chairman John Shewan says the tax is still a hard sell.
''Most capital gains start life as a peacock and end up as a feather duster, because they have quite narrow effects,'' he said.
''It's a very volatile revenue stream that goes up and down depending on the state of the economy.''
Broad coverage tipped
Radio New Zealand's Parliamentary chief reporter says Labour's capital gains tax would be broad enough to cover the sale of businesses, shares and individual assets, as well as investment properties.
However, the family home would be exempted.
Party sources say the tax will be broad based, with some exemptions. It would be levied on any profits made from the sale of a business, shares, property or other significant assets.
It would not be retrospective - the value of the capital gain would be assessed from the time the policy was implemented.
National says the plan would take years to reap any real economic rewards and is a complicated regime for taxpayers.
Radio New Zealand's reporter says Labour will pitch the tax as an alternative way to raise revenue, to National's plan to partially sell state assets.