The Singaporean-owned telco was required by regulation to notify the department about the emergency call outage, which occurred after a routine firewall upgrade on September 18 and was linked to three deaths.
Optus sent two emails to the department on the day, at 2.45pm notifying about the outage and another at 2.52pm advising the matter had been resolved, department deputy secretary for communications and media James Chisholm said.
But the emails were sent to a redundant address, meaning the department didn't learn about the outage until a phone call from regulator the Australian Communications and Media Authority at 3.30pm on September 19, 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," Mr Chisholm said in Parliament House in Canberra on Wednesday.
"We were not notified of the outage properly and, in this case, it was by the regulator, until the Friday afternoon."
Department officials said the email address telcos were required to notify of outages was only changed seven days before the outage, but companies had been warned of the changeover weeks earlier.
In a fiery Senate estimates hearing, frustrated Greens senator Sarah Hanson-Young accused Mr Chisholm of being misleading in his opening statement, in which he said Optus notified the department on September 18 but did not reveal it went to the wrong address.
She grilled Mr Chisholm on why an automated reply wasn't set up for the redundant email address, advising it was no longer being routinely checked.
The outage prevented more than 600 triple-zero calls from connecting in South Australia, the Northern Territory, Western Australia and some parts of NSW.
The calls failed to redirect to another functioning network in the area, even though they are legally required to do so.
Optus' emails on the 18th were also sent to an address of a staffer in the communications minister's office.
The staffer did not relay the information to the department, but the correspondence did not reveal the true extent of the outage.
"I have just been informed that Optus has received reports that some customers in SA and WA are experiencing impacts to triple zero calls," the initial email said.
The follow-up email seven minutes later advised triple zero services had returned to normal and reporting showed only 10 calls may have been impacted.
Minor outage notifications are sent regularly without need for escalation, the estimates hearing was told.
Communications Minister Anika Wells on Tuesday introduced legislation to enshrine a triple-zero watchdog into law and told parliament she was only made aware of the outage on September 19.
Opposition communications spokeswoman Melissa McIntosh accused Ms Wells of misleading parliament and criticised the government for voting down an amendment to double the fines telcos can receive for breaches from $10 million to $20 million.
The triple-zero watchdog reforms passed the lower house, 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 the Australian Communications and Media Authority was insufficient.
Singapore Prime Minister Lawrence Wong expressed his condolences over the deaths to Australian counterpart Anthony Albanese as they met in Canberra on Wednesday.
"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 cooperate with the investigation," Mr Wong told reporters.
