Landlords are being warned not to leave it to the last minute to get insulation installed in their properties.
From Friday, ceiling and underfloor insulation must be retrofitted in social housing where tenants pay income related rent.
All other rental homes must be insulated by July 2019.
Landlords must tell tenants what type of insulation a property has and are being advised to keep records of any upgrades they do.
Sustainability Trust chief executive Phil Squire told Nine to Noon there was a "potential tsunami" of properties needing retrofitted insulation.
He said the government estimate of 180,000 properties was probably on the light side.
"If you leave it til the last year you might be looking at, I guess, a lot of shonky work going on, really high prices, those kind of things."
Paul Coggan, manager of the Ministry of Business, Innovation and Employment's tenancy compliance and investigation team, also advised against leaving insulation retrofitting until 2019.
He said three years sounded like a long time, but it would go fast.
Under the law change, landlords must also fit their properties with smoke alarms from Friday.
The alarms must be 3m from each bedroom door or within each bedroom.
While it's landlords who must install the alarms, tenants will be responsible for changing the batteries.
"Potentially the tenants could be responsible up to $3000 exemplary damages if they interfere with smoke alarms and other means of fire escape and notification," Mr Coggan said.