Treasury has referred the National Party budget release to police and says there's sufficient evidence to indicate its systems have been deliberately and systematically hacked.
It comes after Newshub inquired as to whether any information was hacked.
Treasury secretary Gabriel Makhlouf says the breach is serious, and the matter was referred to police on the advice of the National Cyber Security Centre.
"The Treasury takes the security of all the information it holds extremely seriously," he said on Tuesday night.
"It has taken immediate steps today to increase the security of all Budget-related information and will be undertaking a full review of information security processes."
He says there’s no evidence personal information has been subject to the hack.
Finance Minister Grant Robertson says it's an "extremely serious" matter and is calling on the National Party not to leak any more information.
“We have contacted the National Party tonight to request that they do not release any further material, given that the Treasury said they have sufficient evidence that indicates the material is a result of a systematic hack and is now subject to a Police investigation."
Newshub has contacted the National Party for comment.
What do the released details reveal
The National Party released what it claimed was the details of the Wellbeing Budget earlier on Tuesday, two days before the official announcements.
- National leaks what it claims are Wellbeing Budget details
- Grant Robertson admits some Budget details National released are correct
- National's Budget 2019 drops could be the Government's greatest crisis
Details for 18 policy areas included the Government spending $1.3 billion on defence, $740 million on international aid, $744 million for DHBs and $139 million for forestry over the next financial year.
Budget breaches are extremely rare and when it's happened before, Finance Ministers have offered their resignations to the Prime Minister.
Finance Minister Grant Robertson has admitted some of the information is correct, but said he has no intention of offering his resignation.
The Government says the only people who had access are the Treasury, the printers of the Budget, the Parliamentary Council Office which deals with Budget legislation, and the office of the Auditor General.
The chief suspect is the Treasury, the Government's bookkeepers which put together the Budget.
Secretary to the Treasury Gabriel Makhlouf issued a statement on Tuesday, saying: "Right now we're conducting our own review of these reports and the information that has been published."
He said as far as the Treasury is concerned, "the release of Budget 2019 will proceed as planned this Thursday".
In a phenomenal move, Prime Minister Jacinda Ardern refused to express her confidence in Treasury two days out from the Budget.
When asked if she had confidence in Treasury, she said: "As I say, we haven't looked into this in any great detail yet."
And the Finance Minister's trust is wavering.
"This is something we'll now investigate to see if that confidence is maintained," Robertson said.
When asked if heads will roll because of the information being released, he said: "That's all for after Thursday."