Analysis from ASB has found that Statistics New Zealand's estimate that non-citizens accounted for only 3 per cent of house purchases is well short.
Economists from the bank say the figure more likely sits between 11 and 21 per cent.
Examining transfers or purchases of New Zealand property, Mark Smith and Nick Tuffley analysed the information and reached different conclusions to those released by Statistics NZ.
"The estimates suggest that non-resident purchases made up around 3 per cent of nationwide house purchases over the March 2018 year. This was in a similar ballpark to previously published figures. However, anywhere from 11 per cent to 21 per cent of reported purchases over that period involved a non-NZ citizen, with the proportion considerably higher for some areas, particularly Auckland and Queenstown," they wrote in their latest Economic Note.
Kim Mundy, a fellow ASB economist, said the assumption was that the true figure was at the lower end of the 11 per cent to 21 per cent range "but there's no way to know. We are assuming it is nearer 11 per cent," she said.
Parliament is considering legislation which will block people living overseas from buying existing New Zealand homes.
The report spelled out the situation.
According to the Statistics New Zealand figures, around 33,000 houses were transferred in the March 2018 quarter. Of those, just 3.3 per cent were to non-residents, compared to 2.7 per cent over the March 2017 year.
"However, a further 8 per cent of purchases were made by New Zealand visa holders (but not NZ citizens) and 10 per cent by corporate entities, leaving the remaining 79 per cent of purchases to be made by at least one NZ citizen.
"The citizenship status of corporate buyers is unknown, but we suspect that proportionately more would be headed by NZ citizens. As such, the proportion on home transfers to non-NZ citizens would likely be closer to 11 per cent than 21 per cent over the March 2018 year," the report said.
Trade Minister David Parker said the new data showing one in five central Auckland houses were being sold to offshore buyers vindicated the Government's coming ban on foreign ownership.
Parker said foreign investment was highest in the regions where housing was least affordable. Just over 3 per cent of house sales in New Zealand for the first three months of the year went to non-resident, overseas-based buyers but the rate of foreign purchases was higher in Auckland, at 7.3 per cent, and in Queenstown, at 9.7 per cent.
Bindi Norwell, REINZ chief executive, said this month the figures were not high enough to support a foreign-buyer ban, despite higher numbers in some regions.
*Correction: This story previously stated 'non-residents' accounted for only 3 per cent of house purchases. It was, in fact, meant to say non-citizens. The error has been updated.