But Key said the Government wasn't forced into action by the RBNZ's comments.
"It's an ongoing thing. They dovetail in together," he said of the central bank's speech and its action in last week's financial stability report to implement new, higher loan to valuation ratio lending restrictions on residential houses bought inside the Auckland Council geographical boundary.
"The fact he gave the speech didn't make us go and do these things. I don't think that's the right order. But it's fair to say we've been having discussions with them and thinking about these things. In the end what happened was that we got to the point where we said, 'look we're either going to do something in the budget on housing or we're not and if we're going to do it, we'd better get on with it."
With final budget decisions required about a month ago, and "sustained price increases quite a bit quicker than we would have liked", Key said the Government also "started having discussions with the Reserve Bank about the speech they gave. What would be some alternatives? They came up with the LVRs for investors. We thought this would add to and complement that," Key said.
He gave the impression the IRD was not a major supporter of introducing the new "bright line" test, which means proceeds from a property sold within two years of purchase will be treated as taxable income unless it's the family home, an inheritance, or a sale caused by a relationship breakdown.
Advice in 2010 proposed a five year bright line, but IRD did not favour that and had recommended a two year test, against preference among Cabinet Ministers for a three year test, said Key, who said the Government did no polling on the issue prior to its decisions.
Opposition parties have leapt on the tax announcements as evidence of a belated U-turn on the Government's part, and an effective admission of a housing bubble fear in Auckland, where the median house price rose 17.7 percent in the year to April, according to the latest Real Estate Institute of New Zealand figures, with most of that increase since last September's general election.
Key said he did not expect the measures to have so much impact on house buying behaviour that it would either "deflate" the Auckland housing market or put the brakes on demand for new homes or apartments bought "off the plan" and often resold.
However, it was clear very large numbers of New Zealanders were "either deliberately or otherwise very ignorant of the tax law" as it currently stands with respect to income from trading in property and the latest changes should change public understanding.
The tax announcements were "no silver bullet" for limiting Auckland house price inflation, but were expected to have some impact. RBNZ governor Graeme Wheeler said last week its new LVR rules for property investors should take between 2 and 4 per cent out of current house price inflation figures for the city.
The decision to require non-residents to hold both a New Zealand IRD number and bank account was as much to comply with new international money-laundering rules as with slowing down house price inflation, although it would also provide "perfect information" about the amount of residential property being bought by non-residents - a measure that he warned would include New Zealanders living offshore and buying property at home.
