A property data analyst believes thousands of house sales could be caught by the Government's new tax on gains made from selling investment properties within two years.
CoreLogic said the highest proportion of those were in Auckland, where 15 percent of homes sold last year had been owned for less than two years.
The new tax measure, which takes effect in October, is aimed at helping cool the rapid rise of Auckland house prices.
CoreLogic's figures show the rate of homes resold in Auckland within two years was double that for the rest of the country, where fewer than 7 percent sold in that period.
A senior analyst, Nick Goodall, said that was a clear sign of speculation by buyers in Auckland.
"You know, at the moment they have fairly good confidence they'll get that capital gain, and over a short period rather than a longer term - which is generally what the feeling would be when you buy a property," he said.
Mr Goodall estimated the tax could bring in up to $70 million a year, depending on the role of renovations in reducing the taxable part of any gains.
Of 86,000 homes sold in New Zealand, 40 percent are estimated by CoreLogic to be investment properties - with 3400 of those estimated to have been resold in under two years.