Fire Rescue Victoria is investigating the cause of five system outages last month, which lasted up to seven hours at a time.
The United Firefighters Union says the outages are risking public safety, a claim FRV strongly denies.The Station Turn Out (STO) system, used to dispatch crews to emergencies failed, with firefighters receiving phone calls from dispatchers asking if they had received notices to attend emergencies.
The notices had never arrived.
Firefighters and dispatchers had to rely on fallbacks and workarounds like manual phone calls, pagers, and pen and paper to respond to emergencies.
The union says one outage occurred in late April, followed by the five in May.
The same system also provides emergency tones throughout stations, delivers routing information to firefighters and advice on hydrant location.
In its absence firefighters relied on a combination of Google Maps on their personal phones, Melway street directories, and separate apps.
The union said neither the outages nor resolutions were communicated by management to firefighters -instead they became aware through communication between stations or a call from Triple Zero Victoria.
In a letter sent to Commissioner Gavin Freeman seen by AAP, United Firefighters Union Secretary Peter Marshall said the failure of the critical systems put firefighters and the community at risk.
"The failure of these systems has potentially grave impacts," Mr Marshall told AAP on Sunday.
"Firefighters are resourceful in protecting public safety, but these failures make that job more difficult and dangerous."
On Sunday, Fire Rescue Victoria confirmed it had experienced "intermittent technical issues" since May 24.
"We are working closely with Triple Zero Victoria and external experts to investigate the underlying cause of these STO/Firecom outages and resolve the ongoing issue," a spokesperson said in a statement.
"When STO/Firecom outages occur, FRV has contingency plans in place which ensure there is no impact to emergency response.
"At no time has public safety been compromised and the community should continue to contact Triple Zero (000) as usual in an emergency."
Victorian health minister Mary-Anne Thomas said cyber attacks were a real and present threat to both the private and public system.
"It's important that all our public services have contingency plans in place should there be an incident, and that's what FRV implemented," Ms Thomas told reporters on Sunday.
Questions still remain over the December 2022 cyber attack.
The hack significantly affected Fire Rescue Victoria's technology and communications systems and exposed sensitive information.
It also left the Firecom firefighting information system offline for a year, and affected the rostering system for much longer.
