National is casting doubts on the feasibility of Labour's housing plan.

The Prime Minister has revealed a plan to re-classify residential housing under the Overseas Investment Act as "sensitive", and stop those without citizenship or residency from buying existing homes here.

Australian citizens will be exempt, but other foreigners will face scrutiny from the Overseas Investment Office at any attempt to buy.

READ MORE: Govt - Housing market should be a Kiwi one, not an international one

Opposition finance spokesman Steven Joyce said there's absolutely no detail in the plan, and that's concerning.

"A lot of decisions are yet to be made. Definitional decisions, how it would work, all those sorts of things which suggest to me that its not ready for prime time."

Steven Joyce also urged caution when it comes to negotiations around the Investor Disputes clause in the TPPA.

READ MORE: Banning foreign buyers is just 'political grandstanding and xenophobic'

"To suggest that this is an issue is, I think, Labour trying to meet a political need based on what commitments they made before the election, not actually an economic need," he said.

He added that not all the Free Trade Agreements are safe under the proposed plans.

"I think there's real difficulties with it under the South Korean arrangement because of this requirement to treat each other country's investors equally."

In a press conference announcing the plans yesterday, Jacinda Ardern said the Government will only have to re-negotiate one deal, the Closer Economic Partnership with Singapore.