New Zealand's largest insurer has launched a marketing campaign to encourage people to understand the changing nature of home insurance.

A web site, went live yesterday and tells people how their policies change from unspecificed replacement costs to specificed sum-insured cover, meaning people will need to specify the maximum amount that will be available for payment if there was a total loss.

AA has had the new system in place since the start of this year but IAG brand customers will see changes on renewal in the next 12 to 14 months. IAG's brands are State Insurance, AMI and NZI and it also underwrites some home insurance for BNZ, ASB and The Co-operate Bank.

IAG is the largest general insurer in New Zealand and its brands are responsible for about 55 per cent of the country's home insurance market.