Optus was required by regulation to notify the Department of Communications about the emergency call outage, which was linked to three deaths, on Thursday September 18.
The Singapore-owned telco sent two emails to the department on the day, one 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 it was called by regulator the Australian Communications and Media Authority (ACMA) at 3.30pm on Friday, 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 told a Senate Estimates hearing 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."
The outage was caused by a routine firewall upgrade to the Optus network and prevented more than 600 triple-zero calls from connecting in South Australia, the Northern Territory, Western Australia and some parts of NSW.
The calls also failed to redirect to another functioning network in the area, even though they are legally required to do so.
On Tuesday, Communications Minister Anika Wells met with Optus chief executive Stephen Rue and introduced legislation ratcheting scrutiny on the sector by enshrining a triple-zero watchdog into law.
As the Senate hearing was ongoing, in the House of Representatives, the opposition launched a push for an inquiry into the outage, arguing a review being conducted by ACMA was insufficient.
Opposition communications spokeswoman Melissa McIntosh said the government had displayed "disgraceful" behaviour in refusing scrutiny on the triple zero network by not calling an independent inquiry.
