The telco failed to provide access to the emergency call service for 2145 people during the course of the outage, an investigation by the Australian Communications and Media Authority found.
It subsequently failed to conduct welfare checks on 369 people who had tried to make an emergency call during the outage.
ACMA Chair Nerida O'Loughlin said the size of the penalty reflected the critical nature of the breaches.
The government will work with the regulator to prevent future outages. (Mick Tsikas/AAP PHOTOS)
"Triple zero availability is the most fundamental service telcos must provide to the public," she said.
"When an emergency call fails to connect there can be devastating consequences for public health and safety."
Optus failed in the management of its network in a number of areas and the outage should have been preventable, she said.
The 12-hour outage, on November 8 last year, left 10 million individuals and businesses unable to make or receive calls, or complete transactions. A software upgrade was found to be the cause.
Other failings by Optus during the outage were identified in a government-commissioned post-incident review, led by former Deputy Chair of the ACMA, Richard Bean.
The government agreed to a comprehensive set of regulatory and other actions to improve the performance of the entire telecommunications sector when dealing with network outages.
These include requiring better communications by telcos with their customers and other stakeholders during an outage, greater oversight of the triple zero ecosystem, regular systems testing, and ensuring emergency calls can be seamlessly and consistently carried by other carriers when needed.
"Beyond the penalties announced by the ACMA, the Optus outage has directly led to changes for industry regulatory obligations in relation to emergency call services," Ms O'Loughlin said.
"This will require further actions and investment by telcos, including Optus, to provide better safeguards for consumers and enhance the triple zero ecosystem so that Australians can have even greater confidence they will be able to get through to emergency services when they need it."
Minister for Communications Michelle Rowland said the government would continue to work closely with the regulator, industry and other stakeholders to implement the remaining recommendations of the Bean review as quickly as possible, to better prevent and manage future network outages.
In September, a Senate report found that Optus's offer of 200GB of data in compensation for the 12-hour outage was "disingenuous" because few of its customers were likely to make use of the additional data.
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