The security breach was detected in an email gateway system called Barracuda, which supports some IT systems for the territory government.
ACT Special Minister of State Chris Steel said investigations were under way to determine what data may have been accessed during the breach.
"The investigation has now identified that a breach has occurred and a harms assessment is under way to fully understand the impact specific to our systems," he said in a statement on Thursday.
"We are confident that actions taken to date have contained the breach and that there is no ongoing threat."
The Australian Cyber Security Centre has been brought by the ACT Cyber Security Centre to investigate the incident, along with Barracuda Networks.
Security concerns were first detected in late May by Barracuda after the company identified vulnerabilities in its email security gateway.
Mr Steel said following the notification, the ACT Cyber Security Centre detected that the breach had taken place, leading to a rebuild of the affected system.
ACT residents can continue using government services online safely.
"This incident is a reminder for everyone in the community to be vigilant about their personal cyber security, including monitoring their personal information online for any suspicious activity," Mr Steel said.
4°C