New rules requiring landlords to insulate and heat rental homes will hurt tenants, not help them, says National's housing spokesperson Judith Collins.
The government has released new rules for rental properties, and landlords will be required to install ceiling and under-floor installation that meets the current Building Code standard.
They'll also have to supply heaters that can warm a living room to 18 degrees, plus rangehoods or extraction fans in kitchens and bathrooms.
It's expected to cost landlords up to $7000 and all rentals will have to meet the standard by 1 July 2024, or face penalties.
Each year the government will carry out up to 2000 inspections to check that properties are up to standard.
But National Party housing spokesperson Judith Collins said ultimately tenants will be the losers when landlords start passing on the incurring costs.
While Mr Twyford said in his announcement that the cost of doing nothing is too great for the 6000 children admitted to hospital for housing-sensitive illnesses each year, Ms Collins said landlords will be baulking at yet another cost they're expected to wear.
"Phil Twyford is going to make it harder for landlords to stay in the business of investing in the property market," she said.
"Well we've got capital gains taxes, generally a tax on landlords, it's very difficult to see anything Phil Twyford has done or the government is doing that isn't an attack on residential landlords.''
Ms Collins said more rents will be rising faster, as a result.
She said she expects the already climbing social housing waiting list will continue to skyrocket as more and more tenants look to the state for an affordable home.
"If it (the government) can't get it's act together, then what hope for the mum and dad investor, who are 65, going into retirement, and they don't have spare money just floating around, they've been saving up for their retirement and Phil Twyford's just going to make it harder for them."
Spokesperson for the group Stop the War on Tenancies Mike Butler told Morning Report the cost-benefit analysis doesn't stack up.
Mr Butler said he was gobsmacked to see the minister has opted for the 2008 standard and the 1978 standard has the biggest benefit.
"The minister is asking people to waste money to satisfy his beliefs," he said.
"In many cases the 1978 standard, if it hasn't deteriorated, is still good to go."
He said there's no evidence that a minimum of 18 degrees, or any minimum temperature, is healthy.
"People have had no problem heating themselves to the temperature they want, since forever."
He said tenant advocates and the minister have made "fevered claims".
Housing Minister Phil Twyford told Morning Report about half of all rental homes are way below the standards.
"What we're trying to do is drag the standards up and put a minimum in."
These new rules benefit the largest number of rental homes and is justified by a healthy cost benefit ratio of more than 1.5, he said.
He said insulation doesn't last 40 years.
"I wish we could do it sooner ... but the industry doesn't have the capability to do that many retrofitting and insulation jobs in the period we're talking about."
About 15 to 20 percent of New Zealand's poorest people are living in cold damp houses, he said.
"They have no control over whether or not these houses can be warm and dry."
Mr Twyford said every time a tenancy turns over a landlord will have to certify.