The Australian Communications and Media Authority said in a statement it had begun an investigation into the incident which followed a lightning strike on a Telstra cable between Bowral and Orange in NSW..
The ACMA said it would be seeking information from Telstra to understand the circumstances of the outage.
Telstra has to comply with the Telecommunications (Emergency Call Service) Determination 2009 and the Emergency Call Service Requirements Industry Code C526:2011, the ACMA said in a statement.
The outage also affected Telstra's mobile voice services in NSW, Queensland, Victoria and South Australia between 2.05am and 4.50am AEST on Friday.
The ACMA said the outcome of its investigation would be made available to the Department of Communications and the Arts to assist it in preparing a report to Communications Minister Mitch Fifield.
"In line with its normal practice, the ACMA will not be commenting further until its investigation is concluded," the statement added.