Cyber thieves hacked into the computer system of a national security contractor last year, the Australian government will reveal today.
The intruders had access to the IT network for a long period of time and stole large amounts of the defence supplier's data.
The government's Australian Cyber Security Centre (ACSC) learned of the incident last November and helped end the attack.
Assistant Minister for Cyber Security Dan Tehan said it was unclear who launched the incursion, but the government was not ruling out a foreign government.
"It could have been a state actor, it could have been cyber criminals, and that's why it was taken so seriously," he said.
"We're not 100 percent sure, and that's one of the difficulties of this area."
Mr Tehan did not say how long the intruders had access to the system.
But he said the small business' IT infrastructure was now secure.
"That small business has put in place proper cyber security protections that will hopefully prevent such an incident happening again."
'We're seeing growing sophistication in attacks'
The government would today release the ACSC's annual threat report.
The defence-linked incident was one of more than 700 the ACSC responded to last financial year that targeted infrastructure or other private systems of national interest.
"It is worrying, but the thing that is most concerning is that what we're seeing is a growing sophistication in these attacks," Mr Tehan said.
"[It is] absolutely vital that they are building in cyber resilience to everything that they do."
The ACSC identified a total of 47,000 cyber incidents over the past financial year - a 15 percent increase - and more than half of these were internet scams or fraud.
Major Australian businesses were hit by more than 7283 attacks.