Fire Rescue Victoria's computer dispatch system is offline and firefighters are being alerted to emergencies by mobile phone and radio, with the makeshift set-up likely to be in place for days.
The system, along with phone lines, was shut down in response to an issue detected in the early hours of this morning that could be a cyber attack.
Acting Commissioner at Fire Rescue Victoria, Gavin Freeman, said it wasn't clear what had caused the problem.
"Our process and protocol is close things down as a precaution until we can work out what the issue is," he said.
Other authorities are working with Fire Rescue Victoria to find the cause of the problem.
Mr Freeman said response times and community safety had not been impacted.
"If the Victorian community call triple-0, a fire truck is still dispatched the same way it always has been," he said.
"What we've done in fire stations is ensure that we have firefighters staffing radios and we're dispatching that way, so they'll get a call directly from ESTA or the regional areas of Victoria have their paging systems which are still operating."
Mr Freeman said it was too early to tell if the incident was a cyber attack.
"We're not ruling anything in or out at this point in time, we need to keep an open mind and conduct that investigation very thoroughly," he said.
Premier Daniel Andrews said he had had a broad briefing on the incident and he understood there was no notable impact in terms of firefighting responses.
Fire Rescue Victoria is planning to run on its manual system for the next three or four days.
