The Property Investors Federation supports a Labour Party bill requiring a minimum level of heating and insulation in rental properties.
It says that would be preferable to a more onerous Warrant of Fitness, currently being trialled for state houses.
Labour says most state houses have been insulated, and the main problem with damp, cold houses is now in the private rental market.
The federation acknowledges the responsibility of landlords to provide decent housing. It says the bill gives enough phase-in time to plan for any necessary upgrades.
But Housing Minister Nick Smith said the bill is badly drafted and could result in higher rents for the very people it intends to help.