The Australian Communications and Media Authority alleges Optus breached two legal obligations more than 1000 times during the outage on September 18, 2025.
Access to emergency call services was essential to protect the public, the regulator's chair Nerida O'Loughlin said.
"Australians rightly expect that when they call triple zero, their call will connect," she said in a statement on Thursday.
"The circumstances of this outage meant that did not reliably occur, leaving people unable to connect to potentially life-saving services.
"Giving access to the emergency call service is not optional, it is a fundamental legal obligation and the most important public safety responsibility telecommunications providers have."
Optus failed to give customers who made triple-zero calls access to the emergency call service and to ensure those calls were carried appropriately, the regulator alleged.
The proceedings against the Singaporean-owned company announced on Thursday will be heard in the Federal Court.
The maximum penalty is $250,000 per contravention.Â
The legal action follows the massive outage which went for almost 14 hours and impacted 600 calls, with 455 failing to connect.
An independent review, released by Optus in December, found there were at least 10 mistakes by the company and its contractor Nokia during an upgrade.
It took Optus and Nokia 13 hours to understand the problem, with an Optus call centre not escalating early concerns from five people who said their triple-zero calls weren't working, the report by experienced executive Kerry Schott found.
The watchdog has previously taken action against the telco following another triple-zero outage in November 2023.
The action resulted in Singtel Optus subsidiaries, including Optus Mobile, paying more than $12 million in infringement notice penalties for breaches relating to emergency calls.
"The recurrence of a major network outage affecting emergency calls so soon after the November 2023 outage is a significant concern and one of the reasons the ACMA has decided to take this matter to court," Ms O'Loughlin said.
"The ACMA will not hesitate to take strong enforcement action where alleged telco failures have jeopardised public access to the critical emergency call service."
Optus acknowledged the Federal Court proceedings, adding it would be inappropriate to comment further.
"Our focus remains on building a stronger and better Optus," the company said in a statement to AAP.
"We continue to invest in network resilience, strengthen our systems and processes, build and foster a culture of transparency and accountability, and improve the services our customers and communities rely on every day.
"This work is being supported by our broader transformation program and implementation of the Dr Schott report recommendations."
The court action comes weeks after Telstra suffered a service outage that left train networks in disarray and customers unable to dial triple zero.
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