The policy would make it easier for buyers to get into the market, though, as prices would drop.
The Green Party supports a comprehensive capital gains tax. The Maori Party does not.
Property investor and manager Andrew King said when National brought in changes to the rules around depreciation it caused a $15-a-week change to rents. But it also put off people who had been considering buying rental property.
If Labour were to introduce a capital gains tax, it would eventually have to apply to family homes as well. "It really only makes sense if every asset is covered by it. That should be a big worry for homeowners."
The Green Party looks likely to have a stronger presence in Parliament after the election and King said landlords welcomed its policies around home energy and efficiency.
"National have been pushing insulation - lots of rental property owners have taken up [the subsidy for insulation]."
But he said some rental property owners were wary of the Greens' plans to introduce warrants of fitness for homes, to ensure they reached a minimum standard.
"For things such as minimum bedroom sizes, how do you do that? It's a ludicrous idea, it would add to the cost of rental properties."
Peter Thompson, of real estate firm Barfoot and Thompson, said whatever happened, once the election was over, the market would probably improve. "More people are cautious before the election. Once the election is over, people can get back to reality."