Subject to a few exemptions, all rental properties must be insulated by 2019.

Kiwis in rental properties which need insulation under new rules could face price hikes of $15 a week as a result of the changes, a landlords' organisation says.

Labour says a failure to place heating standards into the rules means they are not enough to help families living in cold homes.

The new tenancy laws, which come into effect on Friday, mean all rental properties must have working smoke alarms by then, and insulation within three years.

FAIRFAX NZ Labour says a failure to place heating standards into the rules means they are not enough to help families living in cold homes.

There must be underfloor and ceiling insulation to 1978 standards in all 60,000 social housing properties by Friday, and all new tenancy agreements must declare the level of insulation.

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JOHN BISSET/FAIRFAX NZ. New tenancy rules coming into effect from tomorrow include a requirement for all rentals to have working smoke alarms.

All rental properties, "subject to a few exemptions", must be insulated by July 2019.

NZ Property Investors Federation executive officer Andrew King said the organisation supported the law changes, which would be "a lot cheaper" than a full warrant of fitness scheme for rentals.

"We think it's quite a good, practical and cost-effective way of improving living conditions for tenants."

King said the insulation requirements would increase rental prices for houses which had none, although many properties already did.

"[The average increase] will probably be around 10 to 15 dollars a week - it won't be massive, but having said that, that's quite a significant amount for a lot of tenants, especially when a lot of them are struggling at the moment."

The Government has estimated around 280,000 rental properties are not insulated to the correct standard, although about 100,000 of those will be exempt from the rules as it is too difficult to insulate them.

SMOKE ALARMS 'A NO-BRAINER'

Smith said the new rules were the most significant changes in tenancy law in 30 years, and would affect more than 450,000 rentals.

The requirement for all rentals to have working smoke alarms was "a no-brainer", as it was estimated last year that more than a quarter - or 120,000 homes - did not.

Smith said another significant change would allow the Government to prosecute landlords whose properties breached basic standards, instead of relying on tenants to take cases to the Tenancy Tribunal.

"The intention is for most cases to continue to be taken up by affected tenants, and the new law further supports doing this by strengthening the protection from retaliatory evictions by landlords."

A new investigations team would focus on tenancies of vulnerable tenants, as well as landlords with multiple properties which did not meet the rules, Smith said.

CHANGES NOT ENOUGH - LABOUR

Labour housing spokesman Phil Twyford said the law change was "long overdue", but it did not go far enough.

"For too long, the worst slum landlords have been renting out houses that are cold and damp and mouldy and a risk to the health of people living in those homes.

"This bill is a small step in the right direction, but the big gap is it has nothing to say about heating - it's no good just insulating a cold damp mouldy home if you've got no way of heating it."