Homeowners who pay to get their house valued on top of insurance bills will be left out of pocket if the estimation is too low - but won't get extra if their premium was based on an overvaluation.
Insurance customers must now say how much it would cost to rebuild their property to the same standard.
Previously, homes were insured for an unspecified full replacement cost. Premiums were based on what the insurance company said the house was worth.
Insurance companies are now providing online calculators to help figure out rebuild costs, but say that those who struggle to use them should hire a professional.
Readers have contacted the Weekend Herald upset and confused by the changes.
One said their insurer's calculator could not cope with the fact their home was half cedar and half plaster, with a calculation for either material varying by about $200,000. Their premiums have gone up by nearly 145 per cent since the Christchurch earthquakes. Another said he recently paid $500 to get his rebuild valued, after finding someone by himself because the bank he is insured with said it was not their policy to recommend a provider.
"There is not enough information out there for people to get the right company to do the valuation and there are going to be cowboys out there who will use this to make a lot of money."
Some firms have set up specifically to perform rebuild valuations after the changes came in.
Insurance Council spokesman Samson Samasoni said the changes were introduced because, following the Canterbury earthquakes, reinsurers insisted on knowing insurers' liability.
If the rebuild costs less than the valuation provided by a customer then an insurer will only pay the rebuild cost and no more.
Mr Samasoni said once a homeowner was confident about their rebuild cost, there was no need to re-estimate it every year.
"Your insurer will be able to advise you of the changes in building costs over the past year when you renew your insurance.
"If you carry out a major renovation, then you will know the actual cost of that and should adjust your sum insured accordingly."
Six and out - for $250
From the devastation of the Canterbury earthquakes to a cricket ball through the window - New Zealanders are paying with more expensive insurance.
Until this year many Kiwis have not had to pay any excess or incur no claims bonus penalties to claim for a broken window.
A Papakura homeowner who contacted the Weekend Herald said he was "bloody angry" to learn glass breakages would incur a large excess when he renewed his policy with ASB Insurance in August.
"I queried it with them, and they said it's universal now with all insurance companies due to the earthquake. The problem is now, if a little toe-rag throws a stone through your window, unless you can identify them, you are down the gurgler $250 or the cost of having the window repaired."
IAG New Zealand underwrites ASB's insurance and owns brands including NZI, AMI and State Insurance. Spokesman Craig Dowling said as a result of recent natural disasters, particularly the Canterbury earthquakes, there had been changes, which differed depending on company and policy.
"Some insurance customers have faced an increase of their general excess on home insurance claims, from a base $250 to $400.
"And for breakages, there has been implemented a $250 excess for glass breakages. They were done as a way to off-set the need for further pre