Optus has revealed what was behind the 12-hour nationwide system outage that impacted 10 million customers and around 400,000 businesses last week.

The telco has admitted it was a routine software upgrade which resulted in changes which made changes to the network which then led to key routers to disconnect themselves from Optus’ core network as a protective measure.

“The restoration required a large-scale effort of the team and in some cases required Optus to reconnect or reboot routers physically, requiring the dispatch of people across a number of sites in Australia. This is why restoration was progressive over the afternoon,” they said in a statement.

“We have made changes to the network to address this issue so that it cannot occur again.”

The company apologised for letting its customers down and for the inconvenience of the outage, which is now the subject of an inquiry.

Optus could be forced to pay up to $400 million in compensation.