Telstra has apologised for a mass network outage that affected up to 8 million customers, vowing to provide a free data day on April 3 as compensation.

Key points: 50 per cent of Telstra's 16 million customers affected

50 per cent of Telstra's 16 million customers affected Telstra apologises and promises free data for customers on April 3

Telstra apologises and promises free data for customers on April 3 Telecommunications expert urges Government to order investigation

Complaints from customers having trouble accessing their mobile network poured in from across the country on Thursday from about 6:00pm.

Calls, texts and internet services were affected for up to four hours for some customers, while the majority were reconnected within about 90 minutes, Telstra said.

Telstra chief executive officer Andy Penn said the outage was caused by extreme network congestion when a large number of services disconnected at the same time and were then reconnected.

The root of the outage was sourced back to an international roaming problem that had flow-on effects in Australia, Mr Penn said.

"I'm sincerely sorry to all of our customers. I know today how much you rely on the network and how much you rely on the reliability of the network," he said.

RMIT University telecommunications analyst Dr Mark Gregory has called for an investigation into the outages.

"Damage to Telstra's reputation will continue to grow if they can't put a cap on these outages," he said.

"Telstra has had problems in the past and had a track record in this regard, so it's about time the Government asked Telstra to provide a full report on what has happened this time so we can start to see ... in the public domain what is exactly is happening with our networks."

Dr Gregory said Telstra had 10 nodes across the country for managing mobile cellular customers and that the systems and design of their network were fragile.

Telstra outage: What went wrong? Connection problem overseas impacting international roaming customers had flow-on effect domestically

Connection problem overseas impacting international roaming customers had flow-on effect domestically 8 million customers disconnected from network and bulk reconnection caused congestion

8 million customers disconnected from network and bulk reconnection caused congestion Issue unrelated to last month's outage, but problem of people simultaneously reconnecting to network was repeated

"Telstra have a lot of work to do to identify and find out what happened," Dr Gregory said.

"We can't afford to have telecommunications going down like this.

"It's now an essential service, so it needs to be deemed as such and there needs to be more investigation into faults of this kind."

About 50 per cent of Telstra's 16 million customers were affected.

As some customers vented their frustration over accessing the Telstra mobile and data networks, others took a light hearted approach and the hashtag #telstraoutage started trending on Twitter.

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While Mr Penn said all customers had full service again, some posted online early this morning they were still without mobile coverage.

Twitter user Megan Lawn posted at 6:30am that she had been without her Telstra mobile service in Adelaide for 12 hours.

In February, Telstra mobile customers experienced a mass outage in multiple states, which the company said had been caused by an "embarrassing human error".

Similarly Telstra offered a free data day to mobile customers, who took full advantage of the offer.

The free day last month opened the floodgates, and customers downloaded the equivalent of 2.3 million movies or 5.1 million episodes of Game of Thrones, Telstra said at the time.

"One outage is not good enough ... two is absolutely not acceptable and all I can say is that I apologise and I am committed to address this and make sure we do absolutely everything we can to ensure it doesn't reoccur," Mr Penn said this morning.

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