Labour Party leader Andrew Little says his party will not support the TransPacific Partnership (TPP) deal if there are no measures to restrict foreign buyers purchasing New Zealand real estate.
Trade ministers from the 12 countries involved in TPP negotiations will head to Hawaii this week to continue working towards finalising the deal.
International trade policy expert Charles Finny told Morning Report that the deal would likely be made by Saturday, New Zealand time.
While Prime Minister John Key said any deal would not include controls on foreign investment, Mr Little told Morning Report this was an issue he would not bend on.
"What is important, before anything is finalised, is that New Zealanders get to know whether or not a future Government is going to have the right to legislate and act in the best interest of New Zealand.
"And if we can't regulate or restrict land sales, whether residential or farmed, to non-residential foreign buyers, then we are selling ourselves down the swanny."
Meanwhile, Oxfam New Zealand said it was unacceptable that the public had been unable to see the text of the proposed trade deal with the agreement possible in just a few days.
Oxfam's executive director Rachael Le Mesurier said it was unacceptable that, at this late stage, the public still did not know what New Zealand might sign up to.
"We have not seen, official, the text of this agreement. And this is the trade agreement that far exceeds any trade agreement that we have ever signed up before - it's not like the China free trade agreement.
"It has a far broader range of rights that it's giving to foreign investors."
Ms Le Mesurier said the deal could affect access to affordable medicines, intellectual property rights and the country's sovereignty.