The hack has impacted most of its IT systems including dispatch, phones, email and an important automated system that opens station doors as soon as a call out is received.
"Preliminary investigations confirm this has been a cyber-attack by an external third party and that FRV systems are impacted," a spokesperson for the fire service said on Friday.
"We have taken decisive action in response to this incident and are working around the clock with specialist cyber security firms and our partners in the state and federal Governments, including the Australian Cyber Security Centre."
The service is still able to respond to triple-zero calls through pagers, radio dispatches and mobile phones.
Extra staff are working in stations to make sure someone can listen to radios at all times.
The attack was first discovered when an IT alarm was set off early on Thursday morning.
That triggered emergency precautions, which forced the service to shut down most of its IT systems.
On Thursday, Fire Rescue Victoria Assistant Commissioner Gavin Freeman said he expected systems to be down until early next week and at that stage was not aware of any ransomware demands.
It comes months after several high profile Australian cyber attacks, including the Medibank and Optus hacks.
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