A dispute is taking place over whether the Trans Pacific Partnership will prevent the Government from banning sales of houses or land to foreigners when details are unveiled next month.
Some of the trade deal's provisions were released when talks concluded on Monday, but the specific rules and their impact will not be known until the full text is released.
Council of Trade Unions economist Bill Rosenberg said one of the areas that needed clarification was whether a future Government would be able to block purchases of residential property or sensitive land by overseas individuals or companies.
A briefing paper released by the Government said non-discrimination rules "would prevent the Government banning TPP nationals from buying property in New Zealand".
Trade Minister Tim Groser was not as equivocal, telling the Herald the matter was "an issue ... of interpretation".
New Zealand International Business Forum executive director Stephen Jacobi said he believed the agreement could allow for New Zealand to restrict house sales.
"I would have thought that since the Government has clearly exempted the Overseas Investment Act as a non-conforming measure under TPP, then it could ... modify the Overseas Investment Office to require such purchases to be screened or declined in certain circumstances."
Banning house sales to foreigners is one of the Labour Party's bottom lines for supporting the TPP deal.
Labour's finance spokesman, Grant Robertson, said his party wanted to reserve the right to legislate in the best interests of New Zealanders.
"We believe ... offshore buyers are pushing New Zealanders out of the housing market ... and we believe this an area where we need to act."
Mr Jacobi said the most important disclosure would be on when tariffs would be reduced and to what extent.
5 things to look out for
Stephen Jacobi, New Zealand International Business Forum executive director
The market access schedule. The full details of how much tariffs are eliminated and when.
The general exceptions and "non-conforming measures". The areas in which NZ cannot be sued by investors, such as public health, environment or overseas investment.
The intellectual property chapter. To make sure there are not software patents or parallel importing restrictions.
Potential restrictions for state-owned enterprises.
Environmental and labour conditions. What will the minimum environmental standards and workers' rights be?
Bill Rosenberg, Council of Trade Unions economist
1. Investor-state dispute settlements and what exceptions there are for public health, overseas investment and the environment.
2. Whether NZ is prevented from banning foreign purchases of sensitive land and houses.
3. The rules on Government procurement, and whether they could limit contractors' employment conditions.
4. State-owned enterprises might be restricted from acting in the public interest.
5. Whether biologic patents are extended from 5 years to 7 years.