The Government has confirmed it will conduct an investigation into Telstra's triple-0 outage which saw emergency calls from around the country go unanswered.

Telstra is under pressure to explain how damage to one cable in regional New South Wales caused the outage in a number of states, and a spokesman for the Minister for Communications Mitch Fifield confirmed it had begun asking questions.

"The Department of Communications is conducting the investigation into the network outage which impacted the delivery of calls to triple zero on the 4th of May," the spokesman said in a statement.

"The Department will seek information from Telstra and assistance from the [Australian Communications and Media Authority] in the conduct of the investigation."

The damaged cable, one of the network's three major links nationally, was cut in the early hours of the morning.

It caused intermittent interruptions to triple-0 services in New South Wales, Victoria, Queensland, South Australia, Western Australia and Tasmania.

Hundreds of traffic lights in Victoria were also affected, and there were reports of disruptions to other telephone services and EFTPOS machines.

The telco later tweeted a photo of the cable pit, between Bowral and Orange, which it said had "significant fire damage consistent with lightning strike".

Telstra said the cable had significant fire damage consistent with a lightning strike. ( Twitter: Telstra News )

But Telstra chief executive Andy Penn said he could not guarantee that was the case. According to the Bureau of Meteorology, the closest lightning strike at the time was about 200km west of Orange.

"All of our evidence suggests it is a lightning strike," Mr Penn said.

"There is physical damage and fire damage to the particular pit where the actual cable is, but obviously we can't prove that.

"Really our focus has been on making sure we can restore the services as quickly as possible.

"To the best of my knowledge, the service is fully restored.

"We had a particular set of circumstances here which has affected some services and for that I deeply apologise."

The pit, 20km east of Orange, suffered physical and fire damage. ( ABC News: Stephanie Gardiner )

Australian Communications Consumer Action Network chief executive Teresa Corbin described the outage as a freak incident combined with failed contingency plans by Telstra to re-direct calls.

"I'm surprised because the reality is that Telstra has in place built in network redundancy which means alternative rerouting should happen," Ms Corbin said.

"Telstra has been caught out in this instance. There needs to be full transparency about what their investigations reveal."

Were you been affected by the outage? Email us your story.

Two-hour delay before back-up systems kicked in

NSW Police Assistant Deputy Commissioner Mark Walton said emergency services realised about 2:00am that calls were not getting through to triple-0.

He said back-up systems kicked in from 4:00am, but he did not know how many calls had been missed in NSW due to the outage.

"We really don't have any numbers on that at the moment," he said.

"We have asked that question of all our agencies. That's something we'll assess."

"The problem is not resolved.

"That (triple-0) service is still operating. However, the reliability of the service is a concern and we have activated our normal emergency management redundancy arrangements."

Sorry, this video has expired Emergency Services say Telstra did not inform them of the triple-0 outage

Queensland Fire and Emergency Services said there was a five-minute delay in receiving a "non-life threatening" triple-0 call from the north coast region, but no further delays had been reported.

Sunshine Coast Police Superintendent Darryl Johnson said there were problems with triple-0 calls coming into the communications centre in Maroochydore, but it did not place anyone in danger.

"Our business continuity plan kicked into place and those calls were redirected to Police Link in Brisbane," Superintendent Johnson said.

"They were talking to our crews on the ground and were able to send them on jobs that way.

"There was no-one in any danger out there when they called triple-0 of not getting a response."

Telstra's group managing director of networks Mike Wright apologised to customers for the outage.

"The network of course is designed to recover from these events by switching to alternate routes and it did for the majority of customers," Mr Wright said.

"But what we did see is that some servers didn't switch over as planned and the main one impacted was triple-0.

Sorry, this video has expired Telstra begins repairs on Triple-Zero network

Issue also affecting traffic lights

Victoria's Emergency Services Commissioner, Craig Lapsley, said the issue appeared to be resolved in Victoria.

"It's back up, we're seeing all calls in Victoria being received," he told ABC Radio Melbourne.

"It's a national system that routes you to your location, so in that sense we were a little lost about how an outage in New South Wales has a big impact across other states, including Victoria."

But the disruption also affected several hundred traffic lights in Victoria.

VicRoads said the affected traffic lights were operating but their normal sequencing could be changed, which may cause traffic delays.

Telstra earlier said the cable was cut at 2:05am, causing "intermittent voice mobile connection interruptions".

"Routers impacted by the cable cut were restored about 4:50am and services progressively returned to normal. A @Telstra fibre repair crew has been dispatched," the telco said in a tweet.

"We're working to restore services ASAP and are sorry for service interruptions," it said.

"Current impact mostly in NSW and some interruptions in Vic, SA and WA."

Gerard Hayes, the secretary of the Human Services Union in NSW, said there needed to be a full investigation into the matter.

"It's quite an unforgivable situation that we find ourselves in," Mr Hayes said.

"People's lives will be put at risk because of this. People ring (triple-0) when they are in a situation that is really dire.

"The Government should be on top of this. The Government stands up for the triple-0 number consistently and tells people [they] should call triple-0 when they have problems.

"Well the fact of the matter is what's the point of calling triple-0 when it's not answering."

New South Wales

NSW Police are experiencing "ongoing intermittent issues".

People should call the Police Assistance line on 131 444 if their triple-0 call fails.

NSW Rural Fire Service had no issues with emergency calls overnight.

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Victoria

Ambulance Victoria is experiencing issues with some calls to triple-0.

"Victorians calling triple-0 who don't get through the first time keep trying," Ambulance Victoria tweeted.

Victoria Police said it had been getting calls all night but was investigating further.

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Queensland

Queensland Ambulance said it had reviewed its call logs and identified 11 patients who had been delayed reaching the call centre.

"Of the affected patients we've contacted, we're satisfied there has not been any adverse impact to their care. We will continue to rigorously monitor the service throughout the day," the QAS said in a statement.

Queensland Police said there had been some issues on the Sunshine Coast but the problem did not appear to be widespread.

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Western Australia

Calls to WA Police are still getting through but some people are having trouble connecting with St Johns Ambulance (SJA).

People who cannot reach SJA on either the landline or mobile can request to speak to police and the police will direct an ambulance to the emergency.

The Department of Fire and Emergency Services was also impacted but is now back online.

South Australia

South Australia Police said it was receiving calls but anyone experiencing issues with triple-0 should call 131 444.

Tasmania

Tasmania Fire Service (TFS) said its direct brigade alarm notification system was affected by the issue.

"If your premise is automatically connected to the fire brigade via a fire alarm system and your fire alarm activates you must call Triple Zero to alert TFS," it said on Facebook.

ACT

Police and emergency services are receiving triple-0 calls. Anyone who cannot get through should call 131 444.