Homeowners are being warned they should not put too much faith in the "improvement value" portion of the council valuations of their properties.

The valuations, used to set rates, are made up of a land value and a value of "improvements", including the house itself.

But that has thrown up some questions in the latest round of revaluations.

In one Auckland case, the $1.025 million CV of a property was made up of $930,000 in land value and just $95,000 in improvements.

The house was insured for $550,000 to rebuild.

In another case, a four-bedroom, three-bathroom, two living room house with a double garage was given an improvement value of $350,000.

Nick Goodall, head of research at property data firm CoreLogic, said the land value was based on the typical cost of purchasing vacant land in the area.

The total valuation was what a property was expected to fetch if it sold on the date the valuation was conducted, based on sales of similar properties.

"The residual is deemed the improvements value, so there is no actual calculation of the improvements on the land."

Across Auckland, 88 per cent of the average increase was land value. Only 12 per cent related to an increase in the value of improvements.

Auckland Council head of rates, valuations and data management Debbie Acott said, as land values generally rose more quickly than capital values, it was common to see improvement values reduce overt time as a quantum amount and as a percentage of the capital value.

She said it was important to note that the valuations should not be used for any other purpose, including as a guide to what sum should be insured in a house insurance policy.

What it would cost to rebuild a property is usually significantly different to the "improvements value".

"The Local Government (Rating) Act 2002 defines how rates can be calculated," she said.

"General rates cannot be calculated using the improvement value, although targeted rates can be. Auckland Council uses the capital value to calculate general rates and does not use the improvements value to calculate any targeted rates."