Triple Zero Victoria's computer-aided dispatch system was plunged into darkness at 12.30am on Wednesday, forcing staff to take notes with pen and paper.
Emergency Services Minister Vicki Ward confirmed the system was not fully back online until 4am after a 90-minute power outage.
The digital outage forced call-takers to rely on pen and paper to dispatch ambulances, police and fire calls but all were connected.
"We're still investigating this but ... all calls were responded to," Ms Ward told reporters at parliament.
Victorian Ambulance Union secretary Danny Hill said the outage was disturbing and put patients at risk.
"The system that crashed is what dispatchers use to identify where all the ambulance resources are across the state," he told AAP.
"When it's not working, the dispatchers are effectively blind to where the nearest ambulance is."
Paramedics described the outage as chaotic, fearing some emergency cases might have gone unnoticed, Mr Hill said.
He said an ambulance crew treating a sick patient on Wednesday night had requested assistance from an intensive care crew but were told the nearest available option were too far away.
The crew made their way to the closest hospital, when there was an available crew about 30km away.
Triple Zero Victoria, formerly known as the Emergency Services Telecommunication Authority, came under fire during the COVID-19 pandemic after 33 Victorians died as triple zero call answering times blew out.
Hexagon and Fujitsu were handed a $253 million contract in 2024 to update the computer-aided dispatch system following a 2022 review by former police chief commissioner Graham Ashton, who recommended the system be overhauled.
The Victorian Ambulance Union says the current system is on its last legs, with its replacement delayed until 2027.
"Our members who work in the control room have said the system is hanging on by a thread, there's glitches and outages," Mr Hill said.
It comes after an outage for Optus customers prevented hundreds of triple-zero calls from connecting in South Australia, the Northern Territory, Western Australia and some parts of NSW.
Three deaths have been linked to the bungle on September 18 when a routine firewall upgrade knocked out emergency calls for more than 12 hours.
Victorian authorities are investigating the cause of the power outage that caused the digital shutdown and why a back-up power supply didn't fire.
Opposition emergency services spokesman Danny O'Brien said the state Labor government needed to be clear on when the next-generation system would be up and running.
Ms Ward conceded the move to manual dispatch "can be slower" but said it remained a "really quick system".
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