ACT's housing spokesperson Brooke van Velden says Finance Minister Grant Robertson has "dropped another bombshell on property investors" after failing to rule out putting limits on how much landlords can hike rents.
Appearing on Newshub Nation on Saturday Robertson said "we'll keep an eye on that" and didn't rule out including caps on rent hikes.
"That's not on our agenda at the moment, but we will keep an eye on what happens," he said.
"What I would say is that under the Tenancy Tribunal rules, if rent rises are above the market rate, then that can be challenged. And if people do start behaving that way, then that may well be."
But van Velden warns Robertson's comment "damages investor confidence" and says he "must rule out rent control".
"Combined with this week's shock announcements, the Deputy Prime Minister's welch on rent control fuels a rocket of uncertainty," she says in a statement.
"Building housing supply is a long-term activity. Building homes takes years, paying for them takes decades. When the Government changes the rules of the game at a whim, confidence suffers and less gets built.
"When the Deputy Prime Minister says the Government might start regulating rent on top of new taxes, it damages investor confidence."
And National leader Judith Collins chipped in on Twitter, questioning the impact Labour's having.
"How many more Kiwis do Labour want living in motels before Labour is satisfied with the damage it is doing to our country?" she asked.
National shadow treasurer Andrew Bayly also weighed in on Twitter.
"Not only has Grant Robertson created an uncertain environment for developers trying to build new houses (see TV3's story last night), he is now upsetting the huge number of Mum & Dad investors with possible rent caps. This follows stricter tenancy laws. Why own a rental?" he wrote.