New Zealand Treasury Secretary Gabriel Makhlouf has said his department has gathered "sufficient evidence" to say it was the victim of a hack.
"Following this morning's media reports of a potential leak of Budget information, the Treasury has gathered sufficient evidence to indicate that its systems have been deliberately and systematically hacked," Makhlouf's statement read.
He said on Tuesday the Treasury had referred the matter to the police on the advice of the National Cyber Security Centre.
"The Treasury takes the security of all the information it holds extremely seriously," Makhlouf continued. "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."
Makhlouf also added that there was no evidence to suggest that any personal information held by the Treasury has been affected.
The news of the "hack" followed the opposition party publishing press releases that responded to items listed in the country's Wellbeing Budget, which are due to be handed down on Thursday.
While National leader Simon Bridges said he was not going to disclose how his party obtained the Budget documents, he was firm that his party had not engaged in any illegal behaviour.
"There has been no hacking under any definition of that word ... there has been nothing illegal or even approaching that," he said in a video published by the NZ Herald.
"We have acted legally, appropriately, without any hacking or anything approaching that by the National Party. Or indeed what Grant Robertson is saying, that's how we've got it, he is wrong."
Instead, Bridges is blaming the document leak on "gross incompetence in Treasury".
"There's this potential talk around cybersecurity and so on -- I was minister in charge of cybersecurity for Bill English and what I know is departments like the Treasury, with big organisations, there are attempts at hacking and so on, if not every day, very commonly. I don't know what the situation with that is, but they wouldn't have called in the police if that was what they were worried about," he said.
The Budget is expected to be handed down as planned on Thursday.