Prime Minister Jacinda Ardern says her Finance Minister is not a liar and no one is linking the alleged hacking of the Treasury with the National Party.

Earlier today National Party leader Simon Bridges said that the party's information about Budget 2019 did not come from hacking or any illegal activity.

He lashed out at Finance Minister Grant Robertson, saying he was "lying" for implying that National's information had come from hacking, and being undemocratic by asking National not to release any more Budget material.

Yesterday National released Budget 2019 information throughout the day, forcing the Government onto the defensive.

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Treasury Secretary Gabriel Makhlouf said that the Treasury website had been hacked, with someone making more than 2000 unauthorised attempts to access Budget-related information.

He did not mention the National Party, and referred the matter to police.

In an interview with Radio NZ this morning, Makhlouf said there was no evidence that National had anything to do with the alleged hacking.

But he said the information the party had obtained appeared to be similar to the information that was targeted on the Treasury website.

Last night Robertson said in a statement that he had asked the National Party not to 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".

Bridges said Robertson had clearly implied that the National Party had hacked Treasury, or had received hacked information.

"What annoys me here, frankly, is that it should be Grant Robertson here right now explaining and front up, because he is misleading New Zealanders. In fact, I would go as far to say he is lying," Bridges said.

Though he would not say how National had obtained the information, Bridges repeatedly said that National had acted legally and appropriately, and National had not done anything resembling hacking, nor had it received potentially stolen or hacked information.

He said the Government was trying to "gag" the Opposition.

"And that is an undemocratic outrage."

Ardern rejected his claim that Robertson was a liar, and said the Government had not implied any link between the alleged hack and National's release of Budget information.

"That link isn't being made by anyone. We are simply saying the Treasury has made the decision and informed us of it.

"This is a matter for the Treasury. They hold the information. They know ultimately what has happened here, we don't. They've made a decision on advice to refer it on the police.

"It has nothing to do with us."

She said the Government was not trying to gag the Opposition, and it was up to National to decide what it wanted to do with any remaining sensitive Budget information it had.

Digital business and internet lawyer Rick Shera told the Herald today that depending on how the Budget information was obtained, any party who had possession of it could be charged with receiving stolen property.

"People who obtain the material from a hacker where they know the hacker didn't have the right to obtain that material could also themselves be in trouble," Shera said.

"You have a look at the sections in the Crimes Act about receiving and it just refers to receiving stolen property, property that's been obtained or stolen. A digital file is property so if you know that the person who's giving it to you didn't have the right to it, then you are receiving."