"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 National Party has acted entirely appropriately. @grantrobertson1 has falsely smeared us 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.