The New Zealand government should follow Australia's lead and stop foreign investors buying existing houses, the Labour party says.

NZ Labour housing spokesman Phil Twyford says Australia requires foreign property investors to build new houses.

Mr Twyford thinks that would be more effective than "some complicated land tax idea" that Prime Minister John Key has floated.

Mr Key on Saturday said a land tax on residential property bought by foreigners may be considered, but he didn't think it was necessary at the moment.

Opposition parties have been calling for action to curb foreign speculators who are cashing in on Auckland's overheated housing market.

They believe the foreign investors are adding to the pressure that's forcing prices up, but until now the government has said their impact is insignificant.

The Greens and NZ First also want action, saying Kiwis are being priced out of the housing market.

The scale of foreign investor activity isn't known.

But it will be in about two weeks, when new information is expected to be published.

Since October 1 last year foreigners buying property have had to register their equivalent of an IRD number with New Zealand tax authorities.

The government insists its not a register of foreign buyers, but it will show the extent of their activity for the first six months since it's been operating.

Labour suspects the government knows what the figures will show, and that's why Mr Key has changed his stance.