Key spoke after Quotable Value's monthly housing report this week showed renewed value surges nationally and in Auckland.
King said life was getting increasingly harder for landlords.
During the past year, they had been "stung" by higher LVR ratios, higher risk weighting requirements and the new Bright Line Test, King said.
"There comes a point when you have to ask, if all these measures aren't slowing house price growth perhaps we are shooting at the wrong target," King said.
King also dismissed concerns about the high number of sales to investors.
"QV states that 46% of sales in their latest report were to investors, but around 42 per cent of all Auckland properties are rentals, so this is not unexpected," King said.
"New Zealand rental properties are mostly owned by accidental investors, who often sell up when circumstances change. For instance they could have transferred for work and rented out their old home before committing to the new location. They may have bought it to provide a home for children attending university or inherited the property and after a while need to sell it. This means that rental properties tend to turn over faster than owner occupied homes and therefore make up a higher percentage of overall sales," King said.
"In addition to investors, QV said that house prices are continuing to be driven by low interest rates and strong net migration. The current levels of migration are the highest ever recorded. When combined with a property supply system that is particularly slow and expensive, this is the main reason why prices are going higher," King said.
Nick Kearney, a North Shore lawyer, also raised concerns.
"This will mean only very wealthy people will be able to afford investment properties - the 1 per cent. The very wealthy will then own a majority of investment properties and rent them to the working class," Kearney said.