Supporters of a capital gains tax slightly outnumber the opponents, a new poll shows.

But there is still a significant degree of uncertainty about the tax measure, with nearly a third of people sitting on the fence.

The Herald ZB Kantar TNZ poll comes ahead of The Press leaders debate tonight, where Labour leader Jacinda Ardern's tax policies are likely to come under scrutiny again.

Labour is reserving the right to bring in a more comprehensive capital gains tax if its proposed tax working group recommends one in the next parliamentary term.

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It could also bring in a land tax if in government, though Ardern confirmed yesterday that land under the family home would be exempted.

The new poll found that 35 per cent of people agreed with a capital gains tax on the grounds that it would treat investments like other income and remove an incentive to invest in property.

That was higher than the 26 per cent of people who opposed such a policy because it would tax what are considered traditional ways to build wealth in New Zealand - housing and small business.

Another 32 per cent of people said they were neutral, because a capital gains tax had both positives and negatives.

Support and opposition to a capital gains tax were consistent across age, gender, and regional groups.

If Ardern becomes prime minister in September, she has promised to set up a tax working group to consider a capital gains tax and other tax measures.

The Green Party is the only party which has a capital gains tax in its manifesto.

A tax working group set up by National when it came into office did not recommend a capital gains tax, saying it had "significant concerns" about how it would be applied and about some of the possible side-effects.

Labour campaigned on a comprehensive capital gains tax in 2011 and 2014 before ditching it when Andrew Little became leader after the 2014 election.

The previous policy was a capital gains tax of 15 per cent on housing and other assets, with an exemption for the family home, retirement savings, and small business assets sold to pay for retirement.

It would have captured houses held in trusts, and would also have captured farms - except for the main residence and the immediate surrounding land.

The proposed tax would not have been retrospective, as it would only have applied to any profits made after the tax was introduced.

Capital gains on property are already taxed in New Zealand if people are intending to make a profit.

National introduced a "bright line test" in 2015 which meant any gains on a house resold within two years was taxed, regardless of whether the seller intended to make a profit.

If elected, Labour wants to extend the bright line to five years.

Tax has been one of National's main lines of attacks on Labour during the election campaign, as its says Labour's plans are vague and give voters no certainty about what they could be paying after the election.

Ardern was forced to clarify Labour's position on a land tax yesterday. She said that if it was recommended by the working group, Labour would not apply to land under the family home.

Earlier in the day, she had refused to say whether it would be applied to land connected to the family home.

The Herald ZB Kantar TNZ poll of 1000 people took place between August 30 and September 4 and has a margin of error of 3.1 per cent.