Retrospective law changes could be needed to protect previous land sales to foreigners following the High Court ruling on the Crafar farms.
Ministers approved the sale of the 16 dairy farms to China-based Shanghai Pengxin but Justice Forrest Miller on Wednesday set that aside.
He has effectively put a new set of rules in place for the Overseas Investment Office(OIO) which recommended the sale.
Ministers accepted that recommendation and the OIO now has to start again, using the judge's interpretation of the sections of the Overseas Investment Act dealing with the economic benefit of transactions.
Prime Minister John Key says the government is going to get a legal opinion on the status of previous land sales and whether retrospective legislation will be needed to protect them.
"From 2005 every application has been considered on the previous understanding of the law as interpreted by the OIO," Mr Key said.
Asked whether the court ruling could affect the recent sale of Wairarapa farmland to movie mogul James Cameron, Mr Key replied: "Who knows? That's the point."
He later told Parliament the OIO would reconsider Shanghai Pengxin's application under the judge's rules "and it's eminently possible the OIO will come up with exactly the same recommendation and ministers will continue to accept it".
Land Information Minister Maurice Williamson and Associate Finance Minister Jonathan Coleman, who made the final decision on the Crafar farms, say they expect a new recommendation from the OIO in a matter of days.
Milk New Zealand Holdings, a Shanghai Pengxin subsidiary, was earlier this month granted approval to buy the North Island farms for a reported $210 million.
A rival consortium, led by businessman Sir Michael Fay, offered $171.5 million and went to the High Court to overturn the ministers' decision.
Shanghai Pengxin spokesman Cedric Allan says the company was "quite astonished" by the court ruling and still expects the transaction to go ahead.
OIO manager Annelies McClure says the ruling potentially affects only four of the 21 factors for assessing benefits to New Zealand.
"We will apply the approach directed by the court in a new recommendation to ministers," she said.
NZN