"What typically happens is people move down the quality curve and still get access to alcohol."
Alcohol watchdogs said minimum pricing was effective at raising the price of drinks that were high in alcohol relative to its cost - such as cask wine.
But retailers would be able to absorb the cost of a minimum price, so it would be ineffective unless accompanied by excise taxes.
Mr Key said he did not think an excise tax would prevent people from "pre-loading", or binge drinking before entering bars and pubs.
This went against advice from the Law Commission, which recommended a 50 percent tax hike to push up the price of alcohol by an average of 10 percent.
A Herald DigiPoll found that 56.5 percent of New Zealanders opposed a minimum price for alcohol, while 40.6 percent supported it.
The Maori Party has called for the initiative to be introduced in alcohol reforms which are due to return to Parliament later this month.
But the Government did not want to commit to a decision before the ministry finished its research.