Burgess told the Senate Foreign Affairs, Defence and Trade Legislation Committee on 5 April that the attackers had exfiltrated a small amount of data.
Asked whether the ASD had identified whether it was a state actor, Burgess said: "The level of sophistication here leads us to believe it has to be a state actor. That's our assessment. Of course, that could still be just a very, very clever individual, but we think that's highly unlikely."
He said the question of calling out any country for the attack was a matter for the government, once the ASD had presented its assessment.
Burgess told the parliamentary panel that the damage assessment from the intrusion had been completed and the intruder or intruders had been fully evicted.
Questioned about whether any country would be called out for being behind the attack, Burgess replied: "Attribution is a really difficult thing, so tying it down to a particular country, a particular organisation, and perhaps particular individuals, is a piece of work that takes considerable time. Even if we got to that point, whether that got called out or not is a matter for other organisations — the government — not for the Australian Signals Directorate.
"I would say, though, that if you do get to the point of knowing who it is, sometimes you have an option of calling it out, and the Australian Government has taken action previously to call out state actors for known problems – that's an intent to call out inappropriate behaviour.
"Sometimes, however, there might be other reasons why you don't call it out, but I'll end with attribution: to get to the point where you could prove it in a court of law is terribly difficult."
There were competing claims at the time of the breach, with some organisations, like the ABC, blaming China, while Resecurity, a security outfit in the US, blamed Iran.
Resecurity said an Iranian-linked entity had been responsible for the network attack, adding that it had informed the ASD about its findings and claiming the agency had confirmed this attribution.
But the credentials of Resecurity came under question shortly thereafter, with journalists and technical researchers both citing what they saw as irregularities about its approach. The firm had also blamed Iran for a network attack on multinational software company, Citrix Systems.