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Telstra then declined to share the data, pointing to a formal investigation announced on Friday by Communications Minister Mitch Fifield, which will be undertaken in conjunction with the Australian Communications and Media Authority (ACMA). It's understood there have been no instructions from the regulators to keep information private regarding the total number of calls missed.

Mr Fifield has requested a report with details on mitigation arrangements under the Triple Zero contract with Telstra, and how they will be strengthened as a result of the event.

A spokesman for the minister said the department was "seeking information from Telstra and emergency services agencies about any unsuccessful calls to Triple Zero, and making inquiries into the safety and wellbeing of anyone who attempted to make those calls".

Given Telstra is the country's biggest telco, the total figure across all the phone providers is potentially much higher.

A smaller telco - with less than 20 per cent market share for mobiles - told Fairfax Media its own internal records showed over 150 customers had at least one failed call to Triple Zero during the morning hours.

In 2016/17, over 8.5 million calls were made to Triple Zero, an average of about 970 an hour, ACMA data shows. There are peak times depending on the service, including weekends and New Year's Eve, and more calls are made when there are widespread events, like extreme weather incidences. The total figure includes malicious or hoax calls, misdials and non-life threatening or non-emergency calls.

A spokeswoman for the ACMA said it was seeking information to “fully understand the circumstances” of the outage.

If there was a breach, ACMA could launch federal court proceedings with penalties of up to $250,000 available for each breach as well as the prospect of court enforceable undertakings to ensure future compliance.

Telstra receives up to $22 million a year towards operating costs to run the service, excluding the reimbursement of necessary upgrade expenses.

Telstra boss Andy Penn quickly apologised for the outage after it happened, on Friday saying at a press conference in Melbourne he didn't "have the data" of how many calls were missed.

A Telstra spokesman said the investigation was mainly into the failure of the back-up systems in place and the cause of the damage, currently considered to be a lightning strike.

“Our networks team has been working day and night to understand what happened and we are keeping emergency services, ACMA and government updated on our progress,” he said.

“We take our responsibilities to connect people to 000 very seriously and have deeply apologised for the impact to services."