A spokesperson said the network was designed to switch to stand-by equipment in such an event, and this did happen.

"Following the failover, however, a further fault caused an interruption which impacted 4G connections," the spokesperson said in an update at 11.15am AEST. "There is redundancy built into these systems but this did not operate as intended."

The blackout was the third in May, with an outage to its triple-zero service occurring on 4 May after a cable between Bowral and Orange in NSW was cut due to lightning. On 1 May, the telco suffered an outage of its NBN services and 4G services.

The outage began at 10am on Monday, with the spokesperson saying this had been due to unspecified key network equipment failing. This disrupted 4G voice and data services nationally.

"The impact was widespread and with a large number of customers dropping back to 3G there was significant disruption to 3G voice and data services as demand exceeded the capacity of our 3G network," the spokesperson said.

"By around 11:40am we had successfully isolated the impacted network infrastructure and services began to be progressively restored. It took about two hours for full service to be restored after that, although our team was working longer with some government and enterprise customers to help them with their systems recovery.

"We are still investigating the root cause of the software fault. We are working closely with our technology vendors on the specific element of software which triggered the issue. Our teams have worked around the clock to restore services and to investigate why the redundancy in our network did not prevent customer impact, for which we are deeply sorry."

The Telstra share price has taken a battering as a result of these incidents. At noon today, it was down to $2.74.