Labour's proposal to ban foreigners from buying property in New Zealand could cut across international trade agreements and cost thousands of jobs, National has warned.
Labour leader Jacinda Ardern on Sunday said the ban would be imposed by Christmas if her party wins the election.
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It would be implementing a long-held policy to ban foreign speculators from buying existing houses.
If non-resident foreigners want a house here, under Labour they would have to build a new one.
National's trade spokesman Todd McClay says it could raise serious difficulties.
"A ban would cut across a range of existing free trade agreements we have with countries like Singapore, Australia and Korea, and cause considerable difficulty with China," he said.
"These FTAs have non-discrimination provisions that ensure New Zealanders are treated fairly overseas, and in return that our trade partners are treated fairly in New Zealand."
Mr McClay said the jobs of 620,000 Kiwis rely on having competitive access to overseas markets.
"A ban would force New Zealand to renegotiate many of our trade agreements, we would be likely to lose some trade as a result of renegotiations which would harm New Zealand companies."