Telstra said Triple Zero emergency voice calls are returning to normal after New South Wales, Victoria and Western Australian faced disruptions after a cable was believed to be damaged by lightning.
The issue resulted in "intermittent" receipt of calls to the police, ambulance and fire brigade number.
Telstra CEO Andy Penn told reporters the fire damage to the underlying infrastructure caused one of the nation's major routers to fault.
"We have three major links around the country...what it has done is it has triggered a fault which is existing in one of the routers that we source from an international supplier," he said.
"Those routers are about 12 months old and so something in the way in which this issue has occurred in the signalling of that has triggered an unknown fault.
"To the best of my knowledge, the service is fully restored," he added.
Telstra released a statement claiming: "The restoration of the cable in NSW has been completed. E000 services are returning to normal."
During the outage, in NSW people needing urgent help could call the Police Assistance Line - 131 444 - if they couldn't get through on 000.
Other states have similar arrangements, but Queensland was not affected by the disruption.
New South Wales police said they were concerned that callers would not have been able to get through to emergency services during the ongoing disruptions, but said there were procedures in place.
"Under our emergency management here, if we have disasters or other emergencies, we're well-practised, we plan, prepare, and are able to respond. This is a contingency where we've had difficulty with our communications," Police Assistant Commissioner Mark Walton said at a press conference in Sydney.
"We've moved to other arrangements - the Police Assistance Line picking up the 000 calls that can't get through. The police have been able to rapidly ensure we have enough staff to handle those calls, and divert and dispatch calls for assistance to all the agencies."
Emergency services reported issues at around 2am after the cable, running between Orange and Bowral, suffered "significant fire damage consistent lightning strike", according to Telstra.
Initially, South Australia was also affected, but that situation appears to have been resolved after Telstra workers restored the routers impacted by the cable cut.
"We're working closely with emergency services in those states," Telstra posted on Twitter.
