Lawyers say trade agreements could pose problems for the National Party's plans to place restrictions on the sale of shares in state assets to foreigners.
The Green Party is calling for National to release official advice on the issue before the election on Saturday.
National says it would place restrictions on the initial sale of stakes in four state-owned power companies and Air New Zealand so overseas investors could buy no more than 15%.
Several specialist lawyers spoken to by Radio New Zealand say trade agreements signed with other countries, notably China and Singapore, mean such restrictions could be legally challenged.
The lawyers did not want to speak on the record because of the political sensitivity of the issue so close to the election.
But one did say that while such restrictions could theoretically be challenged, it could be difficult in practice to bring a case.
That's because foreign investors would have difficulty proving financial losses on shares they had not actually bought.