An investigation is underway into the early release of details from the Budget by the Wall Street Journal.
File photo: Parliament Photo: RNZ /Dom Thomas
The news site published key information from the Budget an hour before the embargo lifted at 2pm.
In a statement, the Treasury said the Wall Street Journal had agreed to comply with the strict embargo, and was in the lock-up briefing when they breached it.
A Treasury official was proactively informed of the incident just after 1pm when an article containing the sensitive information was published.
Treasury deputy secretary of budget and public services Struan Little said the situation was a let-down.
"We are extremely disappointed with this serious breach of lock-up protocol," he said.
"The embargoed information made available to the 200-odd people who attended the event was both restricted and sensitive."
Andrew Little said there would be consequences for the breach once the investigation came to a conclusion.
"We are working with the Wall Street Journal to clarify and understand the action that led to the embargo being breached.
"Once we are confident that we have established the facts, we will advise the Wall Street Journal of the consequences of their actions."
An internal investigation has also been launched, which will review existing Treasury protocols to ensure they are suitable for future lock-up events.