Victoria's triple-zero dispatch software is "hanging by a thread" after an unprecedented power outage exposed a long-standing flaw in the state's emergency system.

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

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