New data shows there's a subset of Kiwis who are being hit harder than most by rising interest rates.

At least 3600 first home buyers owe more than what their home is worth, but Cotality suspects the number could be higher.

Cotality Chief Property Economist Kelvin Davidson says true number in negative equity is likely to be higher because many bought with a deposit smaller than 20 percent.

"If you live somewhere like Auckland or Wellington, where values are down 25, then you'll sort of be 5 percent in negative equity - which, very quickly...are based on average values. It could be $40,000. something like that. It's not trivial."

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