Finance Minister Michael Cullen must give a personal assurance that there will no further unauthorised releases from his office of highly sensitive documents, says National Party Leader John Key.

He says the blunder, in which two top-secret figures relating to the Reserve Bank's currency intervention in the kiwi dollar were released to the Sunday Star-Times among Official Information Act documents, "would have been devastating in the wrong hands".

"This is a monumental blunder by Michael Cullen's office.

"This information would have put at risk millions of dollars of taxpayer money by revealing how much the Reserve Bank can risk when it intervenes in the currency market.

"Money traders would have known what the Reserve Bank's spending limit was.

"It's a bit like telling your enemy the size of your arsenal and would have rendered future Reserve Bank interventions useless.

"Michael Cullen must first explain exactly how this happened. It is not enough for his office to simply brush it off as 'an administrative error'.

"Then he must explain exactly what he is doing to make sure that when highly sensitive information is asked for there is a control check on what leaves his office.

"It's hard to believe he signed off on this.

"Perhaps he's having trouble handling the pressure of the widespread criticism aimed at him for presiding over the highest interest rates in the developed world."