18/03/2016 8:41 AM AEDT | Updated 15/07/2016 12:51 PM AEST

Telstra Network Outage: CEO Apologises, Offers Free Data Day For Customers

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Andrew Penn, chief executive officer of Telstra Corp., speaks during a news conference in Melbourne, Australia, on Thursday, Feb. 18, 2016. Telstra, Australia's biggest phone company, reaffirmed that the company is on track to meet full-year forecast. Photographer: Carla Gottgens/Bloomberg via Getty Images

Telstra has apologised after its latest national network outage that affected up to 8 million customers.

The outage hit on Thursday around 6pm when customers began experiencing problems accessing their data network.

About 50 percent of Telstra calls, as well as texts and internet services, were affected over a four-hour period.

On Friday morning, Telstra CEO Andy Penn said the cause was network congestion when a large number of services disconnected and were then reconnected.

"It was just the sheer volume of services that were being reconnected at the same time that caused the congestion. And that is what caused the delays in getting people back up," he said.

"I'm sincerely sorry to all of our customers. We know how much people rely on the quality of our network and the reliability as we all do for our mobile devices every day.

"So we are deeply disappointed."

Penn offered a free day of data to affected Telstra customers on Sunday April 3 as compensation for the disruption.

This came after hordes of customers took to social media overnight to express their frustrations.

This is the second time this year that the telco has had to apologise to customers following a mass outage in February that came down to "embarrassing human error".

"My focus is to make sure that we do everything that we can to make sure that we don't have repeat incidents. I am confident we can do that," Mr Penn said.

"What is most important is we do everything that we possibly can to ensure that our customers continue to get the benefit of what is, after all, a fantastic network. We have great coverage and great speeds.

"But it doesn't help us when we let our customers down."