Victorian firefighters' personal information may have been stolen or accessed by a malicious third party during a recent cyber attack.
Fire Rescue Victoria's IT systems, including dispatch, phones and email, were affected when online criminals hacked the service in December.
The fire service on Friday confirmed it had reasonable grounds to believe information may have been accessed or stolen by a malicious third party.
Personal details of current and former employees, individual contractors, secondees and job applicants could be impacted.
"While investigations are ongoing, there is currently no evidence that FRV information has been posted on the dark web," the agency's statement said.
"It is a complex task to identify what information is involved in this attack and FRV has engaged specialists to assist with this analysis."
The Office of the Australian Information Commissioner has been notified of the possible data breach.
"We encourage those who may be affected to remain vigilant with all online communications, validate any information they receive and follow other cyber security tips," Fire Rescue Victoria said.
"FRV will attempt to contact those individuals who have records on file to alert them to these tips and the support available."
For the past three weeks, FRV has received call-outs through mobile phones, pagers and radio dispatches.
The agency insists community safety has not been compromised.
The hack also impacted a system that automatically opens station doors once a call-out is received.