UPDATED 7.29pm: The Prime Minister's adamant his government's latest moves on property speculation don't add up to a capital gains tax.

John Key's announced new measures today that'll see the tax man pay much more attention to Kiwi and non-resident activities around the purchase of residential property for profit.

He says the measures aren't a capital gains tax, rather they remove ambiguities from existing tax law.

"It's not a capital gains tax, I can tell you that for nothing.

"A capital gains tax is a tax that you owe because you made a capital gain. This is about saying, if you brought a property with the intention of selling it, that's the current law, then you have to pay tax."

A U-turn on a capital gains tax is how the Green Party's viewing the Government's latest policy announcement on property taxes.

People on-selling residential properties, excluding family homes, within a two-year time frame are set to have their tax obligations more rigorously enforced.

The Prime Minister's denying the move is a capital gains tax but Green Party co-leader Metiria Turei says he's being tricky with words.

"It is a capital gains tax. It's a poorly thought out version and leaves a lot of loopholes for people to avoid the capital gains tax.

"But it is a capital gains tax and everyone would think so."

John Key says this includes $29 million worth of funding for IRD.

"This is expected to generate around $420 million of additional assessed tax over the coming five years."

He says extra steps will be taken to bolster tax rules on property transactions.

They include extra enforcement funding for Inland Revenue, requiring property buyers to provide a New Zealand IRD number, and requiring non-residents to have a New Zealand bank account in order to get an IRD number.

New tests will also be applied, to gains made when residential properties are sold within two years of being purchased.