National leader Simon Bridges is adamant that his party has acted legally while revealing sensitive Budget documents.
Bridges will not say how National got the information - but insisted today that it was not hacked from the Treasury's website.
Asked if he might have received stolen information, Bridges said National had acted legally and appropriately.
The Treasury revealed today that 2000 unauthorised attempts had been made to hack its website in the past two days.
But Bridges said that National had acted entirely appropriately and the Government was trying to gag the Opposition and conducting a "witch hunt".
He said that was undemocratic of the Government, and the Opposition had not done anything illegal.
The National Party had never done anything like hacking, Bridges said.
Bridges would not say how National obtained the Budget information that it released yesterday.
He said Finance Minister Grant Robertson was smearing National and went so far as to say Robertson was "lying".
"Grant Robertson has made scurrilous false allegations," Bridges said.
He believed Robertson was "bumbling and incompetent".
Robertson needed to say what he meant by linking hacking to the National Party.
"That is a smear on the National Party," he said.
"We have acted entirely appropriately. We have done nothing illegal.
"Grant Robertson has been freaking out over this. They are in a frenzy over this... They then decide to lash out. They are going for it... looking for scapegoats, for people to blame.
"The National Party has acted entirely appropriately... and I think Grant Robertson knows that," Bridges said.
"Right at the moment, he's [Robertson] defaming the National Party ... You are lying.
"He is saying right now the National Party has obtained the information as a result of a hack. He has implied [that] quite clearly. He has no basis to say that. It is for him to front up on that."
Bridges added that he did not intend to release any further Budget information today, which is what Robertson asked of him last night.
2000 attempts to hack Treasury website
Treasury Secretary Gabriel Makhlouf has confirmed the Treasury website was hacked, with someone making more than 2000 unauthorised attempts that appeared to be directed at Budget-related information.
Speaking to Newstalk ZB's Mike Hosking this morning, Makhlouf said there had been "multiple and persistent attempts" to get unauthorised access to the Budget information on Treasury's website.
"We know that over the period of, say, 48 hours, maybe a bit longer, a number of attempts were made to access information that wasn't ready for public release."
However, he could not confirm whether the Treasury website was the source of the Budget information leaked by National. National Party MP Mark Mitchell this morning categorically denied that the leak was the result of a hack.
Until National started releasing information from the upcoming Budget, Treasury had believed its website was secure and had not picked up that it was being systematically hacked.
"Clearly the moment we were alerted we started to look and that's what we found," Makhlouf said.
"We take the security of that information very seriously which is why I decided to refer it to police," he said.
It was not clear whether the hack had come from within New Zealand or outside, but Makhlouf said that would form part of the police investigation.
A review would look closely at the security of Treasury's website, he said.
Police confirmed Treasury had referred the matter to them and said it would be "assessed".
The Politik website has tweeted this morning that "sources familiar with intelligence services" say "the Treasury hack may be separate to the National Party leak and that it may be much bigger and a real security threat. Even suggestions Russia could be involved".
Speaking to RNZ's Susie Ferguson this morning, Makhlouf compared the hack with attempts to break into a locked room.
"Imagine you've got a room in which you've placed important documents that you feel are secure, which you've bolted down, lock and key.
"But unknown to you one of those bolts has a weakness and someone who attacks that bolt, deliberately, persistently, repeatedly, finds that it breaks and they can enter access those papers. That's what's happened here.
"It wasn't an instance of someone stumbling into the room accidentally; it wasn't an instance of someone attacking the bolt and finding that it broke immediately. It wasn't someone who tried not once, not twice but in fact over 2000 times to attack that bolt."
The hack attempts began at around midnight on Sunday, he told RNZ.
Treasury yesterday called in police over what it called "deliberately and systematically" hacked information in tomorrow's Wellbeing Budget.
In a series of press releases yesterday, the National Party revealed what it claimed were details about the Budget.
Following the leak, the Treasury said it had found evidence indicating the information was "deliberately and systematically hacked".
It said it had referred the matter to the police after advice from the National Cyber Security Centre.
Speaking to Newstalk ZB, technology commentator Paul Brislen said the wording from Treasury suggests those behind the leak had "sinister" intentions.
"We talk about a 'deliberate and systematic' hack. This doesn't sound like it's just somebody noodling around," he said.
"[It's] interesting to see them immediately refer it to police. That would suggest there's enough evidence to aid the police in this and get them started down the track of who is responsible."
Last night, Finance Minister Grant Robertson called on the National Party to stop releasing any Budget-related material following the Treasury's claims.
But National leader Simon Bridges fired back and said his party has acted "entirely appropriately".
He said Robertson "falsely smeared [National] to cover up his and the Treasury's incompetence".
"When what has occurred is revealed, he will need to resign."
The hacking of the Treasury website would be a major issue for the Government and Bridges digging his heels in on the eve of the Budget's release would also be hugely problematic.
However, Brislen said when the word hacking is thrown around people often suspect someone behind a computer - but this isn't always the case.
"Hacking's one of those widely misused and misunderstood terms," he said.
"It could be as simple as somebody emailing the contents of a file and that is considered at the lighter end of this activity.
"It could be as detailed as somebody from the outside trying to access systems that they are not allowed to access and finding a way into Treasury's computer systems.
"That's got to be really quite concerning beyond what's going on with this particular hack because if they have found a way in then everybody else is at risk as well."
Brislen added the alleged hacking was "by far" the most severe security breach in terms of the level of intrusion, particularly so close to a Budget.