Labour's proposed changes to property-buying rules would only force landlords to raise rents, investors say.
If it became Government, Labour says it would extend the time properties had to be owned before their sale wouldn't be subject to a capital gains tax - from the current two years to five years.
The party said it would also look at the rules around negative gearing, which it says can be used by speculators to make taxpayers subsidise losses on their properties.
But Property Investors Federation executive officer Andrew King said those changes would lead to increased costs for those renting out houses, not just speculators.
"The bright line test is currently aimed at speculators and traders to make sure they pay their fair share of tax. By increasing the test to five years, it turns it into a capital gains tax on rental property," he said.
He said the proposed changes would significantly increase the costs for suppliers of rental houses and would ultimately force them to increase rents.
"Good tax policy should never influence an individual's investment decision. Making rental property less attractive does just that and can only have a negative impact on the supply of rental property without any extra effect on traders or speculators."