The telco's startling mistake was revealed in a parliamentary hearing on Wednesday, along with the admission it underplayed the severity of the issue.
The Singaporean-owned telco sent two emails to the department on September 18, the same day a routine firewall upgrade knocked out emergency calls for more than 12 hours.
But the emails were sent to a redundant address, meaning the department didn't learn about the outage until a phone call from a regulator at 3.30pm the next day, more than 36 hours after the outage began.
"That communication ... was sent to the wrong address, which we have told industry a number of times is not to be used as a source for notification," department deputy secretary James Chisholm said at Parliament House in Canberra.
"We were not notified of the outage properly and, in this case, it was by the regulator, until the Friday afternoon."
The outage prevented more than 600 triple-zero calls from connecting in South Australia, the Northern Territory, Western Australia and some parts of NSW, and has been linked to three deaths.
The bungle even led visiting Singapore Prime Minister Lawrence Wong to express his condolences over the deaths to Australian counterpart Anthony Albanese as they met in Canberra.
"We expect our companies to act responsibly and we will certainly expect (parent company) Singtel and Optus to comply fully with the laws, to do whatever they can to co-operate with the investigation," Mr Wong told reporters.
While the outage prevented hundreds of calls, Optus originally suggested it was just a handful.
Optus sent its first advisory email at 2.45pm, followed seven minutes later by another email erroneously advising the matter had been resolved and only 10 calls may have been impacted, Mr Chisholm said.
Department officials said the email address telcos were required to notify of outages was changed seven days before the outage, but companies had been warned of the changeover weeks earlier.
Greens senator Sarah Hanson-Young grilled Mr Chisholm on why an automated reply wasn't set up for the redundant email address, to advise it was no longer being routinely checked.
Optus' emails on the 18th were also sent to an address of a staffer in Communications Minister Anika Wells' office.
Ms Wells told parliament she was only made aware of the outage on September 19.
The staffer did not relay the information to the department, but the correspondence did not reveal the true extent of the outage.
Minor outage notifications are sent regularly without need for escalation, the estimates hearing was told.
Legislation to enshrine a triple-zero watchdog passed the lower house on Wednesday, but won't be able to become law for several weeks because the Senate does not sit until late October.
In the House of Representatives, Ms McIntosh pushed for an inquiry into the outage, arguing a review by regulator the Australian Communications and Media Authority was insufficient.
Australian Communications and Media Authority head Nerida O'Loughlin said it was notified about the outages on the 18th but was only advised that 10 calls had been affected.
The team member who took the call from Optus noted 10 calls looked a bit low, given the volume of calls that usually go through.
On the 19th, Optus chief executive Stephen Rue confirmed the significantly higher number of blocked calls.
Senator Hanson-Young slammed the regulator for not being proactive.
"You are the watchdog. It is your job to keep these companies in line and to the law, but you waited for them to come back 24 hours later with the worst news possible," she said.Â
"Was everybody out to lunch on Friday or something?"
An Optus spokesperson said an independent review it commissioned into the incident was expected to be completed by the end of the year.
"We take these matters extremely seriously and remain committed to full transparency and accountability," a statement from the telco said.
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