Emergency call-takers reverted to pen and paper for hours after an unprecedented power outage that exposed a long-running flaw in one state's triple-zero system.

Triple Zero Victoria's computer-aided dispatch system was plunged into darkness at 12.30am on Wednesday.

Emergency Services Minister Vicki Ward confirmed the system was not completely 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.

It comes after an outage for Optus customers prevented hundreds of triple-zero calls from connecting in South Australia, the Northern Territory, Western A

See Full Page